RFC 8118 defines the application/pdf Media Type. It is a freely available publication from the IETF.
RFC 8118 was most recently updated in 2017 by ISO subject matter experts to reflect the transition from a proprietary specification to an open ISO standard with the initial publication of ISO 32000-2:2017 (and to update associated references in RFC 8118). RFC 8118 was also updated to reflect the latest specification of fragment identifiers found in Annex O of ISO 32000-2. RFC 8118 states that the application/pdf Media Type applies to all PDF versions and all PDF subsets (such as PDF/A, PDF/X, etc.).
Fragment identifiers are added at the end of a URI to provide a reference to subordinate content within the target of the URI. These fragment identifiers anchor onto specific content or characteristics of the document to which the URI refers. PDF is just one type of document that can be referenced from a URI and the fragment identifiers that are relevant to a PDF are different from those of other document formats. PDF fragment identifiers enable URIs to open to a specified page in a document, a named destination, specify a zoom level to a region on a page, or search and select for a specific word.