PDF is a flexible format, and using PDF in certain contexts requires additional conventions. For example, PDFs are not accessible by default; they define how characters are placed on a page but do not contain semantic information on the content. However, it is possible to generate accessible PDFs, which use tagging to add semantic information to the document.
Pandoc defaults to LaTeX to generate PDF. LaTeX’s
\DocumentMetadata interface supports PDF standards and
tagging when using LuaLaTeX; set the pdfstandard variable
to enable this (see below). For older LaTeX installations, alternative
engines must be used.
The PDF standards PDF/A and PDF/UA define further restrictions intended to optimize PDFs for archiving and accessibility. Tagging is commonly used in combination with these standards to ensure best results.
Note, however, that standard compliance depends on many things, including the colorspace of embedded images. Pandoc cannot check this, and external programs must be used to ensure that generated PDFs are in compliance.