Probably familiar with the Compare feature in your Word program, but have you ever tried to compare for differences between two PDFs? Not the easiest task, usually. But, as usually is the case, someone has ridden to the rescue with a very cool application called DiffPDF. Open source and available for both Windows and OSX. Once you load up your two docs, you can specify whether you want the application to scan for appearance, characters or words. Differences will then be highlighted in the dual doc window. Compare particular pages or page ranges, or the entire document.
Local and not web-based, so upload security is not an issue. Very handy to have when you need it. Thanks, Lifehacker and QTrac!