How to Add Alt Text to Images in a PDF for ADA and WCAG Compliance

Alternative text in a PDF is not a caption and not a tooltip. It is a property stored on a Figure element in the document's tag structure, and a screen reader announces it in place of the image. If the document has no tag structure, there is nowhere for alt text to live — which is why "add alt text" so often turns out to mean "tag the document first".

Tagging comes first

A tagged PDF carries a structure tree: a parallel description of the document saying this is a heading, this is a paragraph, this is a table, this is a figure. Assistive technology reads the tree, not the visible page.

An untagged PDF is drawing instructions with no semantics. A screen reader can only guess, and guessing produces the familiar experience of a document read out in the wrong order with images skipped entirely.

So the order is: tag the document, then set alt text on the figures. Adding a caption under a picture does nothing for a screen reader user, however helpful it looks.

Not every image needs alt text

This is the part most guidance gets wrong by omission. Images fall into two categories, and treating them the same makes documents worse.

Informative images carry meaning — a chart, a diagram, a photograph being discussed, a logo that identifies an organisation. These need alt text.

Decorative images carry none — background textures, rules, borders, spacer graphics. These should be marked as artifacts, which tells assistive technology to skip them. Giving them alt text means a screen reader user hears "decorative swirl" fourteen times in a document.

A rule that works: if the image vanished and the reader lost nothing, it is decorative.

Writing alt text that is actually useful

Say what it means, not what it is. For a chart, the useful alt text is the finding — "Bar chart: revenue rose each quarter of 2025, roughly doubling from Q1 to Q4" — not "bar chart with four bars".

Do not start with "image of". The screen reader already announces that it is a graphic.

Keep it to a sentence or two. If the image needs a paragraph, it needs a description in the body text that everyone can read, with short alt text pointing at it.

Do not duplicate the caption. If the caption already says it, the alt text repeats into the user's ear. Mark the image as an artifact or write something complementary.

Complex images need a long description. Detailed charts, maps and schematics cannot be compressed into one sentence honestly. Put the full data in a table or in the text.

Tools that can actually do this

Setting alt text requires a tool that edits the tag structure. Adobe Acrobat Pro is the usual route — its Accessibility tools include a Set Alternate Text pass that walks every figure in the document. Some other professional PDF editors offer the same.

Free options are thin on the ground, and this is worth saying rather than pretending otherwise. The most reliable free approach is to fix the source: add alt text in Word, InDesign or Google Docs, and export to PDF with tagging enabled. Both Word and InDesign carry alt text and structure into the exported PDF. Doing it in the source is also faster and survives the next revision, which retagging a PDF by hand does not.

Be clear about this site's boundary too: the editor here changes page content — text, images, layout — on files up to 10 MB and 100 pages. It does not edit the tag tree, so it is not the tool for adding alt text.

Check your work

Automated checking catches the mechanical failures — missing alt text, untagged figures, a filename used as a description. It cannot tell you whether the alt text is any good, because that is a judgement about meaning.

The free PDF accessibility checker on this site runs a WCAG 2.2 AA-relevant, PDF/UA-1-oriented ruleset and reports figures with no alternative text, blank descriptions, placeholders and filenames used as descriptions, and images sitting outside the tag structure with no artifact marking. It will not tell you the document is compliant — nothing automated can.

The judgement half needs a person: read the alt text aloud with the image covered and ask whether you would understand the document.

Frequently asked questions

Is a caption the same as alt text? No. A caption is visible page content. Alt text is a property on the figure's tag, read instead of the image.

What if a PDF is not tagged at all? There is nowhere for alt text to live. Tag it first, or re-export from the source with tagging enabled.

Do decorative images need alt text? No — mark them as artifacts so assistive technology skips them.

How long should alt text be? A sentence or two. Anything longer belongs in the body text, where every reader can benefit from it.

Can I add alt text for free? Most reliably by adding it in the source document and exporting to a tagged PDF. Free PDF-side tagging tools are limited.