An image-only PDF contains non-text content that is not accessible. If a document appears to contain text, but doesn't contain fonts, it may be an image-only PDF. 

To fix this issue automatically, right-click "Image-only PDF" on the Accessibility Checker panel and choose "Fix" from the Options menu. In the "Recognize Text - General Settings" dialog box, click the OK button to perform optical character recognition (OCR) on the document. Alternatively, you can also open the Edit PDF tool to automatically apply OCR to a scanned document. OCR converts images of words and characters to actual text.

A user performs optical character recognition (OCR) on an image-only PDF.

Text that Acrobat Pro does not recognize is listed as an "OCR suspect," or a text element that Acrobat suspects was not recognized correctly, according to W3C. To correct an OCR suspect, open the Accessibility tool and click on the Tags panel on the left-hand side of the window. Click the Options drop-down menu and select "Find." In the "Find:" drop-down menu, select "OCR Suspects." Select "Search Document" to have Acrobat search the entire document and then click "Find." Acrobat will outline OCR suspects in blue. Click the "Tag Element" button and another dialog box will pop up. In the "Type:" field, select the correct tag for the content item. Click "OK" to save your changes and continue addressing the remaining OCR suspects.

A user searches for optical character recognition (OCR) suspects on a PDF in Adobe Acrobat Pro.

WCAG requirements: 1.1.1 Non-text Content1.4.5 Images of Text