As with all 40k fluff pdf troops will be of whatever inconsistent quality the author needs, so you can pretty much let your imagination run wild.
To apply some "real world logic" to pdf troops, they would probably perform mainly garrison duties, manning anti air/orbit defences and key infrastructure, have a localized air/orbit force. Carry out VIP protection duties on planetary officials and government buildings.
I would imagine they would be well trained and equipped in "reality" but would lack heavy equipment in large numbers as ground and orbital defences would probably be expected to do most of the work.
One could also assume they would have their own HRT/SWAT special ops whatever units.