Not all PDFs are created equal. For example, when scanning a paper document to a PDF format. Scanned documents are not readable by screen readers without some accessibility remediation.
Once a document is scanned it needs to be tagged to be considered accessible. Tagging is process of adding computer coding that provides screen readers with the information on what to read and the order to read it.
Adobe Acrobat DC has a Tool called Enhance Scans that recognizes text within scanned files. Begin text recognition by opening the tool and running the Recognize Text > In This File option. Next, open the Accessibility tool and run the Autotag Document option. If there are images, run the Set Alternate Text option (also in the Accessibility tool list).
Now that the scanned document is tagged, you may continue to remediate accessibility errors that appear in the Full Check Accessibility tool and review the Tags panel to ensure proper screen-reading order.