How to keep the dropped PR ?

Is there something to do to keep the PR of a site, if I buy a dropped domain with high PR ? Or would it keep its PR anyway ?

You should research its backlinks and make sure that it has some strong ones still pointing to it. The best way to preserve existing backlinks is to build a new site that's got similar content to what the site used to be (check archives.org etc.) so it's not so obvious that the domain's got a new owner.

You can preserve it and get high pr if you do consistent backlinking on that link.Why you keep quiet? Keep on doing link building on that link.