What have you been doing with all the PDAs?

I'm not really sure what to do with them. I've played the previous games and there are plenty of times where a seemingly random pda was super useful for a hidden, unmarked quest (and, shit, even the real ending for the first game), so I've just been keeping every PDA, note, research packet, USB drive, etc in my roaming stash box.

Which ones should I hang onto and which are just there for a gag/reference and some cash? I have so fucking many of these things lol