There were two "pure Linux" PDAs that I fully enjoyed using. [1] Agenda VR3. It was a joy to use, but had a few downsides: Batteries went down fast. Even though the PDA was pure Linux, it didn't support Linux on your PC. Screen was "iffy". And their Marketing wasn't good enough, for them to stay around. (I still have mine, but the screen is barely readable).
[2] Sharp Zaurus. Also a joy to use. Unfortunately, I no longer have the hardware, but I think that there is still a User Community for them.
One of my favorite actions, was to Telnet (local network only!) to them, so that I get a large, full screen Shell on an "itsy bitsy" PDA.
Good luck on your search!
Kenneth Parker