The latest PDF 2.0 standard (ISO 32000-2:2020) is now available at no cost. For additional reference, this page provides external links to all legacy Adobe PDF references and errata, as well as to the ISO 32000 family of standards.
https://opensource.adobe.com/dc-acrobat-sdk-docs/acrobatsdk/#pdf-reference - new Adobe resource in May 2022 providing some legacy PDF reference documents only (no errata).
https://www.adobe.com/devnet/pdf/pdf_reference_archive.html - original website with all legacy Adobe documents. No longer available except via the WayBack Machine.
https://www.loc.gov/preservation/digital/formats/fdd/fdd000316.shtml