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. Tagging support in LaTeX is in development and not readily available, so PDFs generated in this way will always be untagged and not accessible. This means that alternative engines must be used to generate accessible PDFs.
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.