[gnufs]

"download in secure PDF format" (from the ebooks.com page linked from feynmanlectures.info/docroot/buy)

Is this some sort of DRM'ed format that's not readable on non-Adobe supported systems?

And, if it is, is there a way to buy a non-DRM version?

[abecedarius]

Good question. I never buy DRM unless there's a powerful reason (and "I really want it" doesn't qualify).