DiffPDF is a simple but effective free Windows utility that identifies changes and differences between PDF files. Visually comparing PDFs to identify changes or differences is harder than it sounds, and it sounds hard enough, not to mention extremely boring. But it's all too easy to miss subtle differences with the Mark I Eyeball, and some changes might not be visible at all. Why put yourself through it when there's a free tool that can do the job for you, only much faster and more thoroughly? It's called DiffPDF, and it's from RubyPDF Technologies. With dual file selection tools and display panes, this efficient tool makes quick work of a tedious but necessary job.

In addition to side-by-side main views and file browsers, DiffPDF has a right-hand control panel that includes a log display as well as access to Help, About, and Options. One neat feature is the ability to pop out each section of controls into separate dialogs that we could drag and place anywhere on the desktop. You can set DiffPDF to compare appearance or text, zoom in or out, and navigate through multiple documents, but most of what this tool does, it does automatically when you press Compare. We opened two random multipage PDFs from our test folder, one in File No. 1 and the other in File No. 2. We pressed Compare, and the tool very rapidly highlighted every difference in each view, recording the details in the log. Next we opened identical copies of a single document. DiffPDF displayed a message in each window stating that the documents appeared to be identical. We made some small changes and reran the tool. DiffPDF identified and logged the new changes.

DiffPDF is a highly specific tool that does a highly specific job, and most people probably won't need it. But if you find yourself editing and comparing PDFs the hard way, you'll appreciate this simple freeware.