I had to build a diffing engine for PowerPoint files in my first start-up. I initially tried to do this at the OOXML level, and one the weirdest things I found about the format is that the GUIDs are mutated by PowerPoint even if there have been no changes. The serialized OOXML is not really canonical and constantly changes as clients open and save it.

The simplest solution ended up being rendering images of the slides (not easy) and doing it with a slider.