Accessible PDFs
An accessible PDF is tagged with a correct reading order, contains alternative text for images and holds real, selectable text rather than scanned images of text, so that screen readers can capture the content reliably.
In practice
A plain scanned PDF with no recognised text is one of the most common and most easily overlooked barriers on the web. Accessible PDFs are produced most reliably by exporting them with the tagging features of the source document (Word or InDesign, for example) rather than simply "printing" a flat scan to PDF. Before publication, it is also worth checking the file with an accessibility checker.