Computer screen reader packages such as Jaws, Window-Eyes, Hal and Supernova depend on a document's structure and descriptive tags, which act as spoken instructions even though they are in text. A blind person or someone with low vision or reading difficulties can then hear the text read in the correct order and within a setting that makes sense.
Adobe Acrobat (version 5.0 onwards) makes it possible to check pdf files for accessibility. You can also see the security settings that may be in place, which protect copyright but may also affect the use of the document with speech.
The latest version of Acrobat Reader (which can be downloaded free from the Adobe website) offers a few accessibility options under the Edit - Preferences - for example you can change background colours.
Magnification is possible in all versions, with good results for real text rather than text made in a graphic, but it is important to have a document that reflows ( i.e. the text wraps round as the magnification increases).
For further help, use training documents available on the Adobe website.
For files made by others, Adobe provides an online or e-mail method for converting PDF files, as well as a download for Acrobat reader with search and accessibility in several languages and other plug-ins to aid accessibility.