PDF's

PDF is a universal file format that preserves the fonts, images, graphics, and layout of any source document, regardless of the application and platform used to create it. Adobe PDF files are compact and complete, and can be shared by anyone with free Adobe Reader software. If your document is a newsletter, for instance, and has been formatted in a certain way that makes it more readable and identifies it as being yours, you may want to publish it in PDF format.

The advantage of PDF is that you can have total control over formatting and not worry about the user destroying your hard work. An example of this might be an application form which has specific fields and shouldn’t be tampered with. The downside of PDF format is that not everyone can access the files. Some folks may not have the free PDF Reader and others may not be able to view the file because of a physical exceptionality. It's important to give users an alternate version of the file, maybe as text or HTML.

View examples of PDF files:

PDF File
PDF Converted to Text (RTF Format)

You'll notice that the Text Version isn't perfect, but with a little editing it can be a fine substitute for the PDF file. Also note that if scanned text (newspaper or magazine article) had been used in the PDF, it wouldn't have converted to text so easily. A scan of an article essentially creates a large image.

WebAIM tutorials on PDF files: http://www.webaim.org/howto/acrobat/