PDFs are especially problematic for accessibility. The PDF file type is simply a container for content. If the content of the PDF is not accessible, then the PDF is not accessible either. Be sure any content you create and save as a PDF is accessible from the start by consulting the accessibility guides for Microsoft Office.
PDFs that you did not create must still be made accessible. Use Adobe Acrobat Professional to ensure content within the PDF is accessible. The following guide provides information on assessing the document accessibility and correcting any issues found.