To be accessible to screen readers, figures and images in PDF documents should not have blank ALT text, except for decorative images which should be marked as artifacts.

Each image should have an ALT attribute describing the picture, which screen readers can read aloud.

EXAMPLE:

Still life with colorful tulips

ALT="Still life photo of colorful tulips in a glass vase"

ADDITIONAL NOTE: When an image contains words that are important to understanding the content, the text alternative should include those words. This will allow the alternative to accurately represent the image.

Read more about PDF Techniques for WCAG: Text Alternatives.