When we talk about accessible PDFs, what we typically mean are PDFs that have been tagged. Tags provide a hidden structure to a PDF (like the headings mentioned in the section of this guide on Word Processing) that help a screen reader navigate the content.

• To make sure that your PDFs are accessible, the first thing you should do is optimize accessibility in your source file. In other words, you should spend time making sure that the original file (whether in Word or some other application) is appropriately formatted with headings, alt-text, and meaningful hyperlink text. 
• Once you have made your original source file as good as it can be, you’re ready to export.
• Make sure that “Enable Accessibility and Reflow with tagged Adobe PDF” is checked when you export your file to a PDF (this is typically the default, but you should double check).