Just because someone rides around with an AR in their car doesn't make it a PDW.
I don't think there's any hard definition, and as with everything definitions/classifications change over time, but I'd argue that a PDW is quite a bit more compact than a CQBR sized AR15. Probably a really good example to demonstrate the difference is that nobody markets a CQBR as a PDW. A PDW-variant of a given weapon's family will always be substantially smaller (LWRC, DD AR15s, HK416 or FN SCAR as examples). Even in SMGs, a PDW will always be a smaller variant (MP5K-PDW vs other collapsible stock MP5 variants).
I'd argue a PDW can have a barrel no longer than 7.5", but preferably shorter, and absolutely requires a collapsible or folding stock. A non-folding or non-collapsible stock is a complete non-starter for a PDW...a regular 4-6 position adjustable stock is laughable as a PDW.