I've been having a discussion about this with Mazryonh recently, and I have some misgivings about using it as a category for weapons.
The main issue is it's extremely poorly defined. PDW was a marketing term invented by H&K to sell a version of the MP5K with a folding stock. It's since been used to refer to everything from totally normal SMGs (Jatimatic, PP2000) to compact assault rifles (Magpul PDR, XM8 Compact). Granted, there's a "proper" meaning (SMGs firing special AP ammunition), but for every example there's two things called PDWs that don't qualify.
As a result, when pages are listed by weapon type, it allows compact assault rifles and machine pistols to end up in a section that's supposed to be for SMGs. Our own
Category:Personal_Defense_Weapon includes four 5.56mm compact assault rifles, the QCW-05 which is available in two pistol cartridge chamberings, and the PP2000 which is a 9x19mm SMG. It makes it hard to say "this is in the wrong category" when we're using a term that means whatever a gun company's marketing department feels like it means.
I suppose we could define exactly what we want it to mean somewhere, but it's really no good as a category if you have to tell people what it means before they understand why only some things called PDWs can go in the PDW section. I personally favour just getting rid of it and putting things in either the pistol, SMG or rifle categories.