The Wikipedia article says that Pathfinder is intended to be backward-compatible with 3.5e:
It extends and modifies the Revised 3rd edition Dungeons & Dragons (D&D) game rules published by Wizards of the Coast under the Open Game License. Pathfinder RPG is intended to be backward-compatible with Dungeons & Dragons edition 3.5.
I assume "backward-compatible" means I can take 3.5e content and use it in a PF campaign. But what about vice versa? What I've found so far is that 3.5e monsters with the same name and CR tends to have lower stats in PF. For instance, see Ankheg (PF/D&D), Dire Rat (PF/D&D) or Owlbear (PF/D&D).
Can I use monsters from the Pathfinder SRD in a 3.5e campaign without changing their Challenge Rating, or does it cause balancing issues by making encounters too easy?