PDPct is something we also figured out a while back... I don't really recall the details.
IIRC, it's a formula that starts at 82.5, and then grants a small addition for good stat outcomes (interceptions and passes defensed) and a small subtraction for bad stat outcomes (completions allowed) all on a per-pass-play basis.
That is why a lot of defensive linemen, rarely in coverage, will have ratings very close to 82.5 (which can make them look "good" when your typical starting DB might have a rating of 76 or so). They have very few of any of those events to move their number far from the origin.
So, PDPct is just an amalgamated number of stats that we already see and nominally understand.
(now, the related question would be how much do the coverage ratings for the defensive player tagged as "allowing the reception" matter in the dice-rolling that determines whether the play is successful? that's the black box, but most assume there's a "coverage cloud" concept at work, where the overall effectiveness of your players on the field contribute to that outcome, not just the one player.)