Alternative text — "alt text" — is the short written description a screen reader speaks aloud in place of an image. For someone who can't see the picture, that sentence is the picture. Get it right and a blind reader gets the same information a sighted reader gets at a glance; get it wrong, or leave it out, and the image becomes a silent gap or an unhelpful "image" announcement. This guide shows you how to write alt text for PDFs that is concise, useful, and correct.
Why alt text matters
Roughly one in four U.S. adults lives with a disability, and many rely on screen readers to use documents. When a screen reader reaches an image in a tagged PDF, it looks for alt text. If meaningful alt text is there, it reads it. If it isn't, the reader hears nothing useful — and may miss the chart, the signature, the diagram, or the call-out that the rest of the document depends on.
Alt text is also a baseline requirement under the accessibility standards covered in our WCAG 2.2 and PDF/UA pillar guide. WCAG's "non-text content" requirement and PDF/UA both expect every meaningful image to carry a text alternative. It's one of the most common things automated checkers flag — and one of the easiest to get partly right and still get wrong.
Informative vs decorative images
Before writing a single word, decide what kind of image you have. This choice determines everything.
- Informative images carry meaning: a chart, a photo that illustrates a point, a diagram, a screenshot, a signature, a logo that identifies an organization. These are tagged as
Figureand need alt text. - Decorative images add visual polish but no information: a background texture, a divider line, a stock photo behind a heading, a repeated brand flourish. These should not get alt text. Instead, mark them as artifacts so the screen reader skips them entirely. (Artifacts are explained in our guide to PDF tags and reading order.)
Putting alt text on a decorative image is a real mistake: it forces the reader to listen to "decorative swoosh, blue gradient" with no payoff. When in doubt, ask: if this image vanished, would the reader lose information? If yes, it's informative. If no, it's decorative.
How to write concise, useful alt text
Good alt text describes the image's purpose in context, not every pixel. A few principles:
- Convey the meaning, not the appearance. Describe what the image tells the reader, given where it sits in the document.
- Be concise. Aim for roughly a sentence. If you need a paragraph, the image is complex — see the next section.
- Don't start with "image of" or "picture of." The screen reader already announces it as a figure. "Image of a bar chart…" becomes "Figure, image of a bar chart." Just describe it.
- Match the context. The same photo of a building might be "The downtown branch office" in a directory or "Accessible step-free entrance on the north side" in a facilities guide. Purpose drives wording.
- Don't duplicate a visible caption. If a caption already describes the image and the screen reader will read it, repeating the same words in alt text is redundant noise. Provide complementary detail or keep the alt text brief.
Complex images: charts, graphs, and diagrams
A data chart can't be summarized in one line without losing the data itself. The standard approach is short alt text plus a longer description nearby:
- Short alt text states what the image is and its headline takeaway — e.g., "Bar chart of quarterly revenue; Q4 is roughly double Q1."
- A longer description provides the underlying detail. The most robust place for it is in the surrounding text of the document itself — a sentence or short paragraph that walks through the data. This benefits every reader, not just screen reader users, and it survives any tool.
A useful test: if the chart were removed, could a reader reconstruct its point from the text around it? If not, the data isn't truly accessible yet. For tabular data specifically, consider presenting the actual numbers as a real, tagged table rather than only an image — see accessible tables in PDFs.
Special cases
- Logos. Use the organization or product name as the alt text — "Acme Corporation logo" or simply "Acme Corporation" when it functions as the name. Don't describe the colors or shapes.
- Icons. Describe the function or meaning, not the glyph. A phone icon next to a number is "Phone:" not "telephone handset drawing." If an icon is purely decorative and the adjacent text already conveys the meaning, mark it as an artifact.
- Infographics. These are complex images. Give short alt text for the overall point, then make the full content available as readable text or a structured breakdown in the document — never trap the information inside the graphic alone.
- Mathematical formulas. In a PDF/UA-1 file, describe the equation in alt text using clear, unambiguous language (read it the way you'd say it aloud). The newer PDF/UA-2 standard supports native MathML so equations live in the structure itself; our PDF/UA-1 vs PDF/UA-2 explainer covers what that changes.
Common mistakes to avoid
- Starting with "image of" / "picture of."
- Missing alt text on a meaningful figure (the most common failure).
- Putting alt text on decorative images instead of marking them as artifacts.
- Redundant alt text that just repeats a visible caption.
- Vague filler like "chart," "graphic," or "logo" with no actual information.
- Stuffing keywords into alt text for SEO — it doesn't help search and it harms the reader.
A quick do / don't list
| Do | Don't |
|---|---|
| Describe the image's purpose in context | Describe every visual detail |
| Keep informative alt text to about a sentence | Write "image of…" or "picture of…" |
| Mark decorative images as artifacts | Put alt text on decorative images |
| Pair complex charts with a longer text description | Hide the only copy of the data inside an image |
| Use the org/product name for logos | Describe a logo's colors and shapes |
| Convey what the reader needs to know | Repeat a caption word-for-word |
Alt text rewards a moment of empathy: read your description aloud and ask whether it would give you the same understanding the image gives a sighted reader. If it does, you've written good alt text.
Key takeaways
- Alt text is the text a screen reader speaks in place of an image — for many readers, it is the image.
- Decide informative vs decorative first: informative images are tagged
Figureand need alt text; decorative images should be artifacts with none. - Write concise, contextual descriptions of an image's purpose — skip "image of," don't duplicate captions, and don't keyword-stuff.
- For charts and complex graphics, pair short alt text with a longer description in the surrounding text so the data isn't locked inside the picture.
- Handle logos, icons, infographics, and formulas by case — name logos, describe icon function, and expose complex content as real text or (in PDF/UA-2) MathML.
