I owned in the 90s the Psion 3, 3a, 3c and I really loved it, it fit in pocket and you could sit down and type notes or even walk with it and type with your two thumbs.
I ducked out of the Psion at the 5 as it was getting big, I went with a Refurbished Revo but then they moved from AA batteries to built-in batteries and that killed the product’s lifetime, also by then I was given a work laptop which could do so much and then get into Smartphones like the Treo 650 then Nexus, OnePlus. But I have some nostalgic memories.
Do you want a PDA back now in the 2018 era?
I am kinda doing similar with a 8" Android tablet and 8" BT keyboard so I can tap on screen keyboard when moving and use the keyboard when sat, it works well.