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