Definition
Tagged PDF
A PDF carrying an invisible structure tree (headings, paragraphs, tables, figures in reading order) that screen readers navigate. Tags define how a PDF reads, not how it looks.
Tags are PDF's retrofitted meaning layer, standardized by PDF/UA. Every other accessibility feature depends on them: alt text attaches to Figure tags, reading order is the tag sequence, navigation walks the tag tree. 'Document is not tagged' is the most common failure in every audit because most generators never write tags.
Related tools & guides
Check whether your PDF is accessible
Ceereo audits any born-digital PDF against PDF/UA and WCAG machine checks for free, then writes the missing accessibility layer into the same file. The document looks exactly the same afterwards.