PDC is a distributor ( Publishers Distribution Corp.), that goes back to the Golden Age. All Archie's from this era have the I.D. I do recall reading somewhere that it was thought it might stand for Published in the Dominion of Canada, and had myself repeated that information, but it is along the same lines as the ANC ( American News Corporation) and IND ( Independent News Distributors), other magazine and comic book distributors whose stamp appeared on comics in the 50s and 60s ( ANC went belly up in '57, but has its stamp on Archies from the early 50's)
Ahhh, very interesting, thank you!