I am surprised no one has mentioned the FN P90 - the original response to the NATO PDW requirement.
The original NATO PDW called for a weapon to arm the support troops and vehicle crews, that can penetrate CRISAT ( a NATO simulation of the Russian armour made of kevlar and titanium plates) out to 150m.
Before FN 90 and later the HK MP7 PDW became available, HK kinda made the "PDW" moniker famous in the 90's when they released the MP5 PDW for US air crews, eseentially a MP5K with a folding Choate made stock.
So basicly, a PDW is something you tug under the seat and pull it out when the truck starts burning ( or you are being attacked by rabid dogs while washing the truck), as envisioned by NATO.