PDF Image Alternatives

For reference only -- not part of a11ybot's automated checks.

What It Is

In a tagged PDF, an image's text alternative lives in the /Alt entry of the /Figure structure element in the tag tree. When a screen reader encounters a /Figure, it announces the contents of /Alt; if /Alt is missing, it announces "Figure" or "graphic" and nothing else -- the same failure mode as an <img> with no alt attribute in HTML. For glyphs whose visual form should be read as different text (a decorative drop cap, a stylised ligature, an image of a mathematical symbol), the /ActualText entry replaces what the screen reader speaks. Purely decorative images must be tagged as /Artifact so they are excluded from the reading order entirely, analogous to alt="" on HTML <img>[1].

Why It Matters

The failure is mechanistic, not rare. A Word document with an image that has no alt text in the source exports to PDF as a /Figure tag with no /Alt entry; the screen reader announces "Figure" and moves on, and the reader has no way to recover the content. A decorative divider image that was never artifacted produces a "Figure" announcement between every section, breaking reading flow without conveying anything. A chart exported as a single flattened image with a one-word /Alt ("Chart") technically passes a tag-presence check and still leaves a blind reader with no access to the data. And a scanned report with no OCR layer is, to a screen reader, a sequence of image pages containing zero text -- there are no glyphs to tag in the first place.

How It Relates to WCAG

This is the PDF binding of WCAG 1.1.1 Non-text Content[2]. EN 301 549 clause 10.1.1.1 applies the same requirement to non-web documents, and ISO 14289-1 (PDF/UA-1) specifies the concrete mechanism -- /Alt on /Figure, /ActualText for replacement glyphs, and /Artifact marking for decoration[3][4][1].

Practical Implications

  • In Word, right-click the image and choose Edit Alt Text to write a description, or check Mark as decorative so the export writes the image as an artifact. The source-document metadata is what the PDF export reads -- fixing alt text in the PDF after the fact is strictly rework.
  • In InDesign, set Object Export Options > Alt Text per image (or pull from an XMP field) so the export to tagged PDF carries /Alt through.
  • In Acrobat Pro, use the Reading Order tool to edit alternate text on existing /Figure tags, or open the Tags panel and change a decorative /Figure to /Artifact so it drops out of the reading order.
  • Run OCR before tagging any scanned document -- there is nothing to describe until the glyphs exist as text.
  • For complex charts and infographics, write a short /Alt that names the chart and give the detail in body text or an appendix the reader can navigate to. A long /Alt buried on a /Figure is not navigable.
  • Use /ActualText only for the narrow case of glyphs whose visual form should be spoken as different text (drop caps, scientific notation rendered as images, stylised ligatures). Do not use it as a substitute for /Alt on informative images.
  • A nearby visible caption does not substitute for /Alt. The screen reader announces the /Figure independently of surrounding text; without /Alt it still announces "Figure" and nothing else.

Related Clauses

Sources

  • ETSI EN 301 549 v3.2.1, clause 10.1.1.1[3]
  • Accessibility Standards Canada, EN 301 549 clause 10 (non-web documents)[4]
  • ISO 14289-1 (PDF/UA-1)[1]
  • WCAG 2.2 Understanding 1.1.1 Non-text Content[2]