what is compatible is that almost nothing was removed from the 3.5 version, but the mechanics sometimes changed. They tried very consciously to make it an additive process, so that if you were bringing a 3.5 character over you would have to rejigger as much.
SO, for example, monks still flurry, paladins still lay on hands, wizards still specialize in schools, BUT monks flurry has slightly better to-hit bonuses, paladin's laying on hands provides additional bonuses, wizards get domain like powers from their school specializations in addition to extra spell slots. In general classes got more stuff so to speak.
Some things in pathfinder work very differently, and some 3.5 material will not convert very well, but most of it does. For example, grappling got reworked, lots of spells changed slightly so that they were less "save or die" and are now more"save or suck".
That said, you can generally take a 3.5 splat class and use them immediately, although many people like to make conversions to follow more closely the design aesthetic of the core classes in PF.
So, it's compatible but my advice is to stop using 3.5 material and stick with Pathfinder.
I agree with this.
Anything from 3.5 works just fine with Pathfinder -- but to me, starting fresh in a system that feels like 3.5 but no longer has (for example) Divine Metamagic is part of the draw.