It appears that Pandoc generates PDF documents via LaTeX. One problem with this is that, as far as I can tell, LaTeX can't generate tagged PDFs. This is an accessibility problem. Granted, for documents that are heavy on math and/or graphics, the point is probably moot. But many technical documents that are distributed as PDFs would benefit from being tagged.
Luckily, LibreOffice can produce tagged PDFs. And unoconv is a convenient utility for doing this from the command line. So you can use pandoc to convert to a format that LibreOffice can consume, then issue a command like this:
Pandoc can convert into ConTeXt which can produce PDF/A (tagging included) easily.
Why this can't be done in one command like with xelatex, wkhtml2pdf and what else is supported, I don't know.
Many programs can be used to create PDFs but the quality of output isn't always the same.
Luckily, LibreOffice can produce tagged PDFs. And unoconv is a convenient utility for doing this from the command line. So you can use pandoc to convert to a format that LibreOffice can consume, then issue a command like this:
I've tried it, and it works.