system
Given the following post, respond as if you're commenting on Reddit:
human
[WH40K] How powerful is the PDF?

How do they differ from the Imperial Navy and Imperial Guard?

Have they ever held out against the enemies of the imperium?

Are there any notable PDF forces?
gpt
They're the primary military force/security on an individual planet. Many Guard regiments recruit from their highest/most experienced ranks. 

They can stave off particularly small scale conflicts, like pirate raids or riots, but if an Ork Warband, a Chaos Incursion, or a Tyranid Hive Fleet shows up, they're the first ones to be violently destroyed/eaten/enslaved/sacrificed. 

Can't really think of any notable PDFs post-Heresy, but the Prospero Spireguard was a militia/peacekeeping force comprised entirely of normal humans from Prospero, homeworld of the (traitorous) Thousand Sons Legion.