This is based on an article from the National Center on Disability and Access to Education, describing how properly structured / tagged PDF documents can be made accessible, and the disability types that can be aided by the process:

Accessibility challengeDisability type(s)Solution(s)
Only true headings and lists will convey semantic meaning to a screen reader user.Blind
  • Tag all headings so they are true headings (H1, H2, etc.
  • Tag all lists as ordered or unordered lists.
Images must include an alternative description (alt text) to be meaningful to a screen reader user.Blind
  • Add Alternate Text to each document.
  • Combine complex figures and provide Alternate Text.
Complex charts or tables may not contain proper headings, captions or summaries.Blind
  • Tag table rows (<TR>), table headers (<TH>), and table data cells (<TD>).
  • Set the scope of each table header cell (column or row)
Poor color contrast, especially in images and charts.Color blind, Low vision
  • Ensure sufficient color contrast in text, charts and images. One way to verify this is to print out the document on a black and white printer.
Documents with forms that can be filled in on the screen (checkboxes, text fields, etc.) may not be accessible to screen reader users and may not export correctly to other formats.Blind, all users
  • Make sure form elements have appropriate tags.
  • Verify that the form can be completed using common screen readers.
A page may be read out of order by a screen reader. That is, the reading order and the visual order may be different.Blind
  • Check and edit the reading order using Acrobat reading order tools
  • Verify reading order with screen reader
Scanned PDF files that are not converted to plain text will not be accessible to screen reader users.Blind
  • Convert a scanned PDF file into text using an Optical Character Recognition program.
A PDF reader program must be used to view PDF files.All users
  • The Adobe Reader can be downloaded free from the Adobe site.
  • There are a number of free reader programs that are available for several platforms. The accessibility features of these readers vary.
Embedded multimedia may be inaccessible, especially if it is not captioned.Deaf, Blind
  • Make sure embedded multimedia is captioned. This is possible in a PDF file.
  • If necessary, provide audio descriptions for the blind.
Scanned PDF files converted to real text can have numerous misspellings that may only be apparent to screen reader users.Blind
  • Use a spell check to make sure as many words are spelled correctly as possible.
Headers, footers, logos and other content that meant to be ignored may be read by a screen reader on every page.Blind
  • Convert extraneous information to an artifact
  • If appropriate, make the first instance of repetitive information visible and convert subsequent versions into artifacts.