Why is a Word document often preferred by some readers over a PDF document? They are more accessible for more people. Not everyone can see well; can use a mouse, can read English well, can remain focused easily when they read, and not everyone uses assistive technology. And not all PDF documents can be read by screen readers. In a slideshare Tammy Stitz explains some of the issues and solutions. She covers some of the technicalities as well as basics such as colour contrast, reading order and Alternative Text (alt-t). Logical structure, use of headings and placement and attributes of hyperlinks. The slideshare goes on to cover a list of things that need to be checked. Finally you can test the document using PDF Accessibility Checker. There is also such a thing as a PDF Association.