Accessible PDFs: Tags, Reading Order and Alt Text That Matter

A PDF that looks perfectly organised on screen can be complete nonsense to a screen reader, because the visual layout and the underlying content order are two independent things. Fixing accessibility is mostly about making them agree.

How a screen reader reads a PDF

It does not look at the page. It walks a structure tree of tags — headings, paragraphs, lists, tables, figures — in the order they appear in that tree. If the document has no tags, the reader falls back to the raw content stream, which is the order the generator happened to write the glyphs in. For a two-column layout, that frequently means reading across both columns, line by line, producing interleaved gibberish.

The five fixes with the highest impact

1. Tag the document at all. An untagged PDF is the baseline failure; everything else is refinement.
2. Use real heading tags in a sensible hierarchy — H1, then H2, no skipping levels. Screen reader users navigate by heading the way sighted readers skim.
3. Fix the reading order so it matches the visual order, column by column, including sidebars and pull quotes.
4. Add alt text to every meaningful image, and mark purely decorative images as artifacts so they are skipped instead of announced.
5. Set the document language, so the reader uses the correct pronunciation rules.

Tables specifically

A table needs header cells marked as headers and associated with their data cells. Without that, a screen reader announces a stream of values with no indication of which column or row they belong to. For anything beyond a simple grid, add scope attributes; for tables with merged cells, expect to do the association manually.

Alt text that is actually useful

• Describe the information the image conveys, not the image. 'Revenue grew 40% year on year' beats 'bar chart'.
• For charts, put the underlying numbers in a caption or adjacent table rather than cramming them into alt text.
• Do not begin with 'image of' — the reader already announces that.
• Logos: the organisation name is the alt text.
• Decorative rules, borders and background textures: mark as artifact, no alt text.

Forms

Every field needs a tooltip or label that describes what to enter, and the tab order must follow the visual order. A form where tab jumps from the first field to the signature block and back is unusable without sight, even though it fills in fine with a mouse.

Testing without specialist software

Two quick checks catch most problems: turn on a reflow view and see whether the content order still makes sense, and extract the text to a plain file and read it top to bottom. If the plain text reads sensibly, the reading order is broadly right. Then run a formal accessibility check for the tagging details.