MANUAL: add section on accessible PDFs, archiving standards

Supersedes, closes: #8312

MANUAL.txt

@@ -3585,8 +3585,7 @@ Enabling this extension with `context` output will produce markup
 suitable for the production of tagged PDFs. This includes
 additional markers for paragraphs and alternative markup for
 emphasized text. The `emphasis-command` template variable is set
-if the extension is enabled. Combine this with the `pdfa` variable
-to generate accessible PDFs.
+if the extension is enabled.
 
 
 # Pandoc's Markdown
@@ -7182,6 +7181,80 @@ Some document formats also include a unique identifier.  For
 EPUB, this can be set explicitly by setting the `identifier`
 metadata field (see [EPUB Metadata], above).
 
+# Accessible PDFs and PDF archiving standards
+
+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 by
+default. However, it is possible to generate accessible PDFs,
+which use tagging to add semantic information to the document.
+
+Pandoc's default method to generate PDF output is via LaTeX.
+Tagging support in LaTeX is in development and not readily
+available, so PDFs generated in this way will always be untagged
+and not accessible. 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.
+
+## ConTeXt
+
+ConTeXt always produces tagged PDFs, but the quality depends on
+the input. The default ConTeXt markup generated by pandoc is
+optimized for readability and reuse, not tagging. Enable the
+[`tagging`](#extension--tagging) format extension to force markup
+that is optimized for tagging. This can be combined with the
+`pdfa` variable to generate standard-compliant PDFs. E.g.:
+
+    pandoc --to=context+tagging -V pdfa=3a
+
+A recent `context` version should be used, as older versions
+contained a bug that lead to invalid PDF metadata.
+
+## WeasyPrint
+
+The HTML-based engine WeasyPrint includes experimental support for
+PDF/A and PDF/UA since version 57. Tagged PDFs can created with
+
+    pandoc --pdf-engine=weasyprint \
+           --pdf-engine-opt=--pdf-variant=pdf/ua-1 ...
+
+The feature is experimental and standard compliance should not be
+assumed.
+
+## Prince XML
+
+The non-free HTML-to-PDf converter `prince` has extensive support
+for various PDF standards as well as tagging. E.g.:
+
+    pandoc --pdf-engine=prince \
+           --pdf-engine-opt=--tagged-pdf ...
+
+See the prince documentation for more info.
+
+## Word Processors
+
+Word processors like LibreOffice and MS Word can also be used to
+generate standardized and tagged PDF output. Pandoc does not
+support direct conversions via these tools. Pandoc can convert a
+document to a `docx` or `odt` file, which can then be opened and
+converted to PDF with the respective word processor. See the
+documentation for [Word][word-accessible-pdfs] and
+[LibreOffice][lo-pdf-export].
+
+[word-accessible-pdfs]: https://support.microsoft.com/en-us/office/create-accessible-pdfs-064625e0-56ea-4e16-ad71-3aa33bb4b7ed
+[lo-pdf-export]: https://help.libreoffice.org/7.1/en-US/text/shared/01/ref_pdf_export_general.html
+
+
 # Running pandoc as a web server
 
 If you rename (or symlink) the pandoc executable to