I don't think Pathfinder were allowed any usage of DnD trademarked monsters, which are a handful of creatures like for example the Beholder, Mind-Flayers, anything of the few beings DnD hasn't taken directly from folklore.
Among those trademarks are the "fancy" dnd names for demons and devils, Tana'ri and Baatezu, but not the metaphysical structure of the universe itself and the opposition between the different kinds of fiends though. Basically it's safe to say that for every word DnD could reasonably claim to have invented themselves Paizo had to make up something new for their world; so most of it is virtually the same except at times name-changed.
