PDFs are one of the most common choices for posting documents online or sharing documents over email, so understanding how to make sure they're accessible is key. Accessible PDFs are covered in Module 2. The University of Washington is also a good source of information for creating accessible materials, see Creating Accessible PDFs from Microsoft Word.
PDFs that are made by scanning a document and saving to PDF are not accessible unless you take a few extra steps beyond scanning – see Tagged PDFs.
Depending on which program you're using to create a PDF, you'll take different steps to make it accessible. These are easy-to-follow guides from the University of Washington for three of the most common programs:
From Module 2: "Tags allow the author to identify headings, alt text, lists, etc. They essentially give the document structure and allow the reader to more easily navigate the document.
To determine whether a particular PDF is tagged, open it in Adobe Acrobat or Adobe Reader and go to Document Properties (Ctrl + D in Windows; Command + D in Mac OS X). In the lower left corner of the Document Properties dialogue, “Tagged” is either “Yes” or “No.”
To add tags to a PDF, you will need to use Adobe Acrobat DC Pro. Review these instructions for Fixing inaccessible PDF documents."