The ones that matter get locked. The rest will download into your PDA and can safely sell. The journal entries and caches will still be on your PDA when you sell the rest.
S.T.A.L.K.E.R.
Scavengers, Trespassers, Adventurers, Loners, Killers, Explorers and Robbers.
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100% though? I have already found 2 times where I could offer a PDA that wasn't locked to my inventory as a quest item to an NPC for extra dialogue and a reward. Once with Leech (the guy hanging upside down at Elevator in the Cordon) and once with a trader in Zalissya with a PDA of Sidor and some other dude conspiring to ruin his business.
Now, to be fair, I'm not sure you need to physically have the PDA on you to get the options; but I did at the time. It didn't take them away once offered, though.