Who do you put PD on?

Carries. Dodges other carries and assassins crit.

I either put it on frontline tank or a carry. But honestly Id rather use the bow for something else on a carry if I can.

Whoever you need to live the longest or someone who can use it really well like Gnar since he grabs a lot of attention and is much better when its more likely that he doesn't get killed before he transforms. With Dragon Claw on top of it pretty much any unit in the game is very hard to kill.