If you are having trouble viewing a PDF document in Firefox on a PC, try the following:  

1. At the top of Firefox's menu bar, go to Tools - Options
2. Click the Applications tab at the top of the window
3. Type adobe into the search bar
4. Change the action to Use Adobe Reader for the items listed.


If the above still does not allow you to open a PDF in Firefox, download Foxit Reader, an alternative PDF reader and make it your default.
http://download.cnet.com/Foxit-Reader/3000-10743_4-10313206.html