PDF-UX: Bookmarks

Many PDF users are familiar with the tree-like navigation aid commonly known as “bookmarks”. This feature is typically added by authors to assist readers in navigating the document and can be configured in each PDF to automatically display when the document opens. Just like with physical books, readers (end users) may also wish to add bookmarks to their documents.

Although bookmarks can provide end users with easy, rapid access to the content they seek, they are sometimes created in less-than-ideal ways that lead to end-user frustration and reflect poorly on the author or publisher.

This PDF-UX article offers an overview of bookmarks and suggests considerations and general best practices for every PDF author. It does not specifically address PDF/UA (ISO 14289), which has additional file and processor requirements for outlines related to accessibility. However, the principles and suggestions presented here are aligned with improving navigation and access to content for all users, including those with accessibility needs.

Background

Bookmarks are similar to a Table of Contents in that they help users jump directly to any location in a document. However, unlike traditional Table of Contents pages, bookmarks are displayed in a separate navigation pane or via a menu action, allowing their use at any time, while viewing any page.

The technical implementation of bookmarks is achieved via PDF’s Document Outline feature defined in ISO 32000-2:2020, clause 12.3.3.

ISO 32000-2:2020 defines bookmarks as a feature of the file format capability but does not provide any requirements, recommendations, or guidance to authors, publishers, PDF creation software developers (such as developers of office suite applications), or designers and developers of PDF viewers. Accordingly, interface design choices between PDF applications can become distinguishing features and unique selling points.

Bookmarks are widely supported by PDF viewers, at least at a basic level. Accordingly, as with designing easily navigable websites, thoughtful use and structure of bookmarks in PDF files benefit the widest user base. On the other hand, authors who ignore bookmarks, or have broken or poorly structured bookmarks, risk perceptions of “low quality”, user frustration, and damage to a publisher’s brand.

Most PDF viewing software provides some capability to view bookmarks (if present) - often in a “navigation pane” (or simply “pane”), or (on mobile devices) from a “hamburger” menu. These design choices are not defined by PDF’s specification; they are made by each developer and are thus open to innovative, creative, and differentiating solutions.

Bookmarks are also not just for authors. As a user recently noted, end users can also add bookmarks to make their documents easier to use:

“As a heavy PDF user (mainly a reader), I'm always grateful to 2 features of PDF: annotations and bookmarks. The former allows me to take notes in-place; the latter helps me grasp the outline of the document (no matter how long or how complicated it could be), and jump to any section with ease. I also actively create bookmarks to track the reading progress, collect "cheat sheets", and mark sections of different importance. My typical page zooming is "Fit Width" to make characters larger. Bookmarks that jump to precise locations are a real lifesaver: mostly section titles, and sometimes manually picked positions at a high zooming level.”

Although bookmarks are expressed in a tree-like navigation aid to document content or related functionality, bookmarks are not, technically speaking, PDF page content, typically will not be searched, and may not be extracted when copying content from a PDF.

Creating PDFs with bookmarks

Consider including bookmarks in all PDF files as this feature helps all users, including many with disabilities, to navigate regardless of platform. Bookmarks do not add significantly to file size, so there are no downsides… unless the bookmarks are broken or poorly thought out, thus frustrating users! Even single-page PDFs can benefit from bookmarks if they have distinct content sections or features.

Don’t publish a PDF with a single root bookmark at the topmost level (as shown in the screenshot above). This takes up valuable screen space (both vertically and horizontally, since all child bookmarks will be nested) and serves no useful purpose.

Each bookmark should reflect a significant location, such as a heading, but bookmarks can be anything that helps readers navigate or otherwise use the document, as we discuss below.

However, some consideration is warranted given the differences in how platforms and viewers may handle bookmarks.

For longer or more complex documents bookmarks are indispensable. In these cases, good UX implies that authors should ensure that users discover the bookmarks when the document is opened. To manage this, look for “Document Properties” or the equivalent to find settings titled “Initial View”, “Navigation View” or similar.

In addition to the convenience for end users, displaying the bookmarks when the file opens also helps users who are unfamiliar with their PDF viewing software, or who are simply unaware that the author has provided bookmarks.

Desktop PDF software typically presents the author’s options for displaying bookmarks when the file opens together with a default page layout and page:

The bookmark hierarchy

Bookmarks can include a nested hierarchy of entries, and most PDF viewers allow the user to expand or collapse each level in the bookmark hierarchy.

All popular western office suite applications provide the option of automatically creating bookmarks from headings or other styles when exporting to PDF, however this option may be hard to locate or even disabled by default. Automatic creation of bookmarks is convenient in establishing a starting point, but some tweaking may be required if the bookmarks are fully expanded, or too lengthy for a good UX on small screens.

For documents with many top-level “chapters” and deep hierarchies of headings within each, general best practice is to ensure that each lower level of bookmarks is collapsed by default. Some viewers may provide a means to “expand all”/”collapse all”, but this can push the user into constant vertical scrolling, forcing them to respond by manually collapsing/expanding. On smaller screens, these problems are compounded.

When establishing the default appearance of bookmarks, authors should consider the number of clicks required for a typical user to access specific information - much as web-page authors must consider the UX of web pages that require too many clicks or massive scrolling!

It’s up to authors to decide on how bookmarks appear when the bookmark panel is displayed. Authors can set the bookmark hierarchy to display with bookmarks expanded or collapsed at each level. For larger documents, collapsing everything except the top-level (typically H1) bookmarks may be best, with all child bookmarks collapsed at each lower level. Ensuring that the bookmark tree expands gradually ensures that users retain context and avoid vertical “doom scrolling” of the bookmark pane.

TECHNICAL NOTE: Capable viewers refer to the Count entry in each outline item (ISO 32000-2:2020, Table 151)) that defines the starting appearance of the bookmark hierarchy.

Some suites, such as LibreOffice/OpenOffice, provide options to select the expanded (“visible”) bookmarks:

Bookmark text

Bookmark text should be short, as many viewers don’t provide options for resizing the bookmark pane, altering the bookmark text size, controlling the wrapping of bookmarks, etc. If document sections have longer labels they may not work well as bookmarks.

For example…

• Chapter 1 – Introduction
• Chapter 2 – Background

…is quite lengthy, and may not fit into the pane, especially on smaller devices.

Consider that shorter bookmarks such as “Ch. 1 Introduction”, “1: Introduction” or even just “Introduction” may offer a much better experience across a wide variety of screen sizes and viewers. When duplicating document headings as bookmarks, careful use of appropriate whitespace is very important to avoid layout issue (see this example above, where TABs consume significant screen space). Changing the bookmark text does not affect the heading text on page.

Bookmark text can be Unicode, however PDF does not define the font, font size or typeface for bookmark text - viewers may use any typeface at any size. Bookmarks that use unusual Unicode codepoints may also not display correctly if the PDF viewer does not provide a full set of Unicode glyphs in the font(s) used to display bookmarks. Some viewers may also provide limited capabilities to increase or decrease bookmark text size or to save a default configuration, but if users constantly have to manage their bookmarks in order to comprehend and navigate their documents then that is a massive user disservice!

Bookmark formatting

While it’s possible to format bookmark text, not all viewers support formatting, so authors should never rely on this feature. That said…

• The PDF format supports basic formatting (italic and/or bold, via the F entry) of outline text.
• PDF supports the definition of a device-dependent RGB color for each outline item (C entry). However, as modern devices commonly support a variety of light and dark user interface modes, “hard-wiring” bookmark colors can cause display problems or hinder those with visual disabilities.

Keeping things simple is best.

Bookmark functionality

Bookmarks have many capabilities besides navigation; they can trigger a PDF Action or change the view to a given PDF Destination. This capability means a bookmark might do any of the following (subject to PDF viewer support):

• Go to any location in the current document (typically headings, but can be anything);
• Change the current view (e.g. navigate or zoom to a specific area on a page);
• Open a URL;
• Open an embedded or remote PDF document to a particular page;
• Hide or show specific annotations;
• Enable or disable layers (optional content groups) if present in the PDF document;
• Trigger some JavaScript.

Some of this functionality may be confusing or unexpected for users, so be selective in using bookmark capabilities beyond document navigation. Some of the more advanced functionalities may also not work in all viewers or on all platforms or might invoke additional user interaction as cyber-security countermeasures (such as confirming URLs or downloading other documents).

Considering mobile platforms

On mobile platforms, some PDF viewing software ignores the starting appearance of the outline hierarchy in PDFs and always presents a collapsed top-level view. This choice overrides explicit author intentions, which is unfortunate, but mobile developers likely make this choice due to the limitations of small screen sizes and the significant proportion of PDF files with poorly considered outlines.

Given this tendency (at least for now) on mobile devices, there’s even more reason to carefully consider the arrangement of bookmarks in PDF documents and the devices that the audience is likely to use. A limit of about 10-15 visible bookmark items when a PDF is initially opened is possibly a good balance across a range of desktop and mobile viewers; if that’s all the headings in a document then having every bookmark expanded would be acceptable.

More than headings

PDF bookmarks can target any content and are not limited by the application that creates the PDF file. Capable PDF editing software supports the creation and management of bookmarks in existing PDF files, allowing authors and publishers to optimize their bookmarks as they see fit. For example:

• In addition to the traditional table of contents-style bookmarks, reference documents with many tables or figures benefit from, for example, a collapsed level 1 bookmark entitled “List of Tables” whose 2nd level bookmarks provide navigation directly to each table.
  This is far more convenient for users than having to click a bookmark to visit the “List of Tables” content, then scroll to find a specific Table, then click on a hyperlink to that page (if hyperlinks exist), and then possibly scroll or zoom/pan that page to locate the Table.
• For posters, maps, architectural drawings, or other large-format content: bookmarks can cause an area of interest to fit the window without the user having to pinch, zoom and pan, etc.
• For newspapers or magazines with complex multi-page and multi-column layouts, bookmarks can adjust the zoom of a precise page coordinate to ensure content is readable without further effort by the reader.

Some PDF applications can even create bookmarks automatically based on tags in the PDF, the perceived logical structure, or provide an AI assistant to help.

Users add bookmarks too!

People have added their own physical bookmarks to documents since ancient times. An academic or reference bookshelf is normally filled with well-read books and sticky notes serving as bookmarks. Such reader-provided bookmarks provide an easy way to locate content, whether to pick up reading from last time, as a rapid location finder for important information or as a means of sharing ideas with others. Using digital documents should be no different - readers should continue to be able to add their own bookmarks to documents irrespective of format!.

Capable PDF viewing software allows readers to add and arrange bookmarks in the same easy way that web browsers allow for the management of bookmarks pointing to websites. Although PDF bookmarks can support a lot of advanced functionality, the ability to add a bookmark with a single click can be a differentiating feature for users selecting PDF viewing software.

Unrelated to bookmarks, some PDF viewing software can remember the last location read in a previously-opened PDF file. Similar to how modern web browsers can share tabs across platforms or devices to support modern ways of working, capable cross-platform PDF viewers do exactly the same with digital PDF documents.

Software support

Many PDF viewers support bookmarks (outlines), but many don’t do as much as they could to make PDF bookmarks useful to users.

Does this PDF include bookmarks?

Independent of authors and publishers remembering to set the PDF to open with the bookmarks pane displayed (PageMode to UseOutlines), PDF viewers can provide a visual indication that a PDF document contains bookmarks, helping to make users aware that they have an easy, fast navigation option, and aren’t forced to “doom scroll” through pages.

Exactly how this information is represented is not defined in the PDF specification and varies in each software. Some indicate the presence of bookmarks with a green “Go”, checkmark or some highlight color on the bookmark pane icon.

Does the bookmarks pane reflect my location or the last bookmark I clicked?

Most modern word processing software presents a dynamic outline view of the document’s headings. As the user moves through the document, the outline view updates to show the user’s current location.

A common point of user confusion with many PDF viewers is that, unlike word processors, few PDF viewers will automatically update the outline view to represent the currently displayed page. As a result, if a user scrolls after clicking a bookmark, the highlighted bookmark may not change to reflect the user’s current location in the document.

This behavior is entirely in the hands of the viewer developer, as the PDF specification (ISO 32000) does not define viewer user interface requirements.

Conclusion

Bookmarks are a very well-understood metaphor for users spanning the physical world to their use in web browsers and digital documents. PDF bookmarks allow authors and publishers to provide a well-considered experience of the document, optimizing access to content while allowing end users to add their own bookmarks.

To summarize our suggestions for making the most of bookmarks:

• When the document has bookmarks, be sure to set the file to open with bookmarks displayed.
• Avoid a single top-level bookmark with a nested hierarchy - it wastes space and forces unnecessary clicks, creating a poor user experience;
• As when designing a website, consider the number of clicks (navigation steps) a user must do to navigate your document;
• Ensure that bookmarks have appropriate destinations that correspond to the document’s content. Jumping to a specific page is convenient for many books, but possibly inconvenient for a multi-column layout where a zoomed view of a specific area within the page may be more appropriate.
• Avoid or minimize long bookmark titles that might wrap or crop on narrow panes and small screens;
• Do not rely on bookmark color and typeface formatting, as not all viewers support these features;
• For long and deep bookmark trees, ensure that each lower level is collapsed by default so that expanding bookmarks is gradual;
• Even if you aren't the author, feel free to add your own bookmarks as you see fit.

Although bookmarks are a well-known PDF navigation feature, authors and authoring applications often neglect to design them thoughtfully, with both the document and the reader in mind. In the modern PDF ecosystem, the strategic use of bookmarks can make navigating documents across small screen devices and diverse viewers far less of a burden, whereas missing, non-functional, or poorly thought-out bookmarks frustrate users and risk reputational damage to the author or publisher’s brand.