Mozilla Firefox users!
Some Mozilla Firefox automatic updates may cause you to receive a message that it cannot open the PDF documents because they are corrupted. There is actually nothing wrong with the files, instead the update has "broken" the internal Firefox/Adobe Reader interface.
For a quick fix, right click on the link, then select either Open Link in New Tab orOpen Link in New Window. Many times, this simple process will cause Firefox to fix itself, correcting the issue, and thus allowing you to now directly open PDF links again.
If the right-click/open in new tab or new window options didn't permanently correct the problem, you will have to either wait for the next Adobe Acrobat Reader auto update, or perform the re-install manually. (http://www.adobe.com/products/reader.html)