PDF-PDiff is a JVM-based library that for creating Perceptual Diff's between two PDF documents.
A Perceptual Diff is a visual document that compares two documents, and highlights the areas that are different. This is distinct from a typical contextual diff that compares the content of a file. A Perceptual Diff is a great visual aid that allows computers do what they do well -- fast comparisons and calculations -- and allows the humans to get back to the problem of solving the next problem, rather than trying to identify if two rendered images are the same. If you've ever printed out two documents and held them together up to the light to see where they differ, then you've needed a Perceptual Diff tool.
Quite simply because there wasn't a JVM-based solution out there that did this for PDF's. While there are a few solutions for raw images or HTML, finding a solution for PDF documents was another matter entirely.
NOTE: This tool is still under development, although it's getting close. While you're waiting, a couple of interesting links:
-Travis: https://travis-ci.org/mattcorey/pdfpdiff
-Unit Tests (~15 minutes late): http://mattcorey.github.io/pdfpdiff/reports/tests/index.html