A PDC is a Planetary Defence Centre, which is a ground base and assumed to be under a mountain, which is why you get the armour bonus. Getting them into orbit can be tricky
. . . Er, right, . . . I thought it was "Prefabricated Defence Complex" or something.
Aha! I think it's doing crap all because I haven't manufactured any missiles.
[EDIT] Hmm. . still not there.
It has missiles, and a commander, but the defense value for my planet is still given as zero. :/