What is a tagged PDF?
Updated
A tagged PDF carries an invisible structure tree — labels like Heading 1, Paragraph, Table, Figure arranged in reading order — that screen readers navigate instead of the page's raw ink. Tags don't change how the PDF looks; they define how it reads. An untagged PDF is the single most common accessibility failure.
PDF was born as a printing format: it records where ink goes, not what the ink means. Tags are the retrofitted meaning layer, standardized in PDF/UA.
Everything else in PDF accessibility hangs off the tags: alt text lives on Figure tags, reading order is the sequence of tags, heading navigation walks the tag tree.
Tags are why two visually identical PDFs can differ completely for a blind reader — and why remediation can fix a file without changing a pixel.