The PPK is classic, trim and quite compact. It's heavy, though, for it's size, due to its all steel construction. So, if carrying a PPK, PPK/S or PP, I would opt for a holster...belt, shoulder or ankle, depending on circumstances.
The Sig-Sauer P232 (or the earlier P230) is also excellent...it has an alloy frame. However, it's a little larger, overall, then the PPK (about the same size as a PP). Very nice gun...but, for me, a little large for a pocket. Again, I'd opt for a holster on the belt, shoulder or ankle.
Both of the above are "blow back", rather than locked breech designs, and therefore somewhat more susceptible to malfunctions from limp wristing or loads that don't generate enough slide velocity. Pick your ammo carefully and pay attention to grip technique, if you choose this type of semi auto.
I will join the others who, for pocket carry, recommend some flavor of lightweight J frame Smith. My preference would be the 642 or 442. Not too light, enclosed hammer and utterly reliable. The .38 is a better round than the .380, providing you can master the J frames, which, admittedly, are not easy to master.