Hello Friends, I have no experience black hat seo so i want to ask that our PR will down if we use black hat seo.
Depends what you consider black hat. if you do something that you know is against Google's ToS, then yes, you can lose PR and you can be de-indexed. Some people get away with it, you might not. The fact that you're asking, suggests that you don't know enough to do anything a bit 'shady' without getting caught and penalised.
Black hat can be summarised up into two points: a) If you want to risk damaging your sites reputation in order to gain some easy bucks, then do black hat methods b) If you want sustainable results that could lead into a succesful and long lasting business venture online, go white hat White hat is what Google loves. Black Hat is what it doesnt like, and what will most likely get your site removed from the index.