After running it a few more times and cross-checking the skipped files, file size doesn't seem to be directly related. It's skipped some large ones (800kb), but then it'll turn around and skip a small one (30kb) while accepting another (40kb). As for permissions, those appear to be uniform as well.

There just doesn't seem to be a pattern, at least that I can discern, for which files PowerPoint "can't process."