What
PDFs have a number of accessibility requirements. Some – like headings, color contrast, and alternative text – are shared with other document formats; others are more specific to PDFs, including ensuring selectable text, a reading order that is recognized by the program, a language, a title, and tags on non-text, among other things. Because PDFs can require more time to format than other types of documents, a good first step is to evaluate whether providing a PDF is the best choice or if providing the document in its native format might be a better route.
Why
Many of the accessibility factors that are specific to PDFs are to facilitate understanding for users who are accessing it through a screen reader because they cannot see the display. For example, without a reading order, the screen reader may present columns of text out of sequence and cause confusion.