Alternative text (alt text) describes an image for people who can't see it — screen reader users, low-vision users, and anyone on a slow connection. For PDFs, missing or bad alt text is one of the most common accessibility failures, and it's something a validator can detect but only a human can do well.
Why alt text matters
Without alt text, a screen reader announces an image as "graphic" or reads its filename. A chart showing quarterly revenue becomes noise; a diagram explaining a process is invisible. Good alt text conveys the meaning the image adds — the "why is this here" — not a literal description of every pixel. It's one of the six failures we cover in why PDFs break screen readers.
What good alt text looks like
- Concise and meaningful — "Quarterly revenue, Q3 up 18% on new subscriptions" beats "a bar chart with blue and green bars."
- Not redundant — if the caption already says it, use empty alt text (
alt="") rather than repeating. - Never a filename — "image1.png" is worse than nothing.
- No "image of..." — the screen reader already announces "image," so describe the content.
Adding alt text manually (Acrobat Pro)
In Adobe Acrobat Pro, open the Accessibility tool → Set Alternative Text. Acrobat lists figures and lets you type alt text for each, with a checkbox to mark decorative images as artifacts. For complex figures (charts, diagrams), this manual step is where judgment matters most.
How automated remediation handles figures
Automated remediation detects and tags figures programmatically — it identifies which page elements are images and wraps them in the correct Figure tag. But a machine can't invent meaningful descriptions. So automated pipelines flag figures that lack alt text (your validation report lists them), and a human fills in the descriptions for the meaningful ones. Decorative images are marked as artifacts and skipped. This split — machine tags, human describes — is how you remediate at scale without sacrificing quality. See the compliance checklist.
What the validator can and can't do
A PDF/UA validator checks whether figures have alt text (or are marked as artifacts). It cannot judge whether the alt text is good. A document can pass validation with mediocre alt text. That's why alt text is the one item we always recommend a human review on high-stakes documents — financial reports, technical diagrams, anything where the image carries essential information.
Start with a check
Run your PDF through the free checker to see how many figures lack alt text. That's your work list — and auto-remediation handles the tagging while you add the descriptions that matter.