seems when an item have pd , it will depreciate , especially in asn's items.

its useless in everyones items not just asians

I mean , if there are 2 items.their stats are the same but one has poison damage another one don't. will the PD one depreciate?

I mean in PvP

usually assasins use venom, which lowers poison dmg from regular sources (items/charms, that is) in a specific way:
https://www.theamazonbasin.com/wiki/index.php?title=Venom_(skill)

so if two items are identical with the only difference being that one adds poison dmg, the one with poison will still be better ofc (since some dmg beats no dmg). but if the poison dmg mod is interchanged with something useful on another item, the other item is usually straight up better

If u use poison dagger sin the psn will scale to that duration.

I'm sorry but I lol'd