Windows/Mac: Comparing two different versions of a PDF file can be a nightmare. The open source DiffPDF automates the process, finding differences in both text and appearance.
All you need to do is feed the application two different PDF files, specify whether you want to compare appearance, individual characters or whole words, and let it run. A few moments later, a side-by-side window will highlight any discrepancies in red. This is obviously not an application that you'll be using every day, but it's a huge timesaver for when you do need it.