As other people have noted, Paizo posts their entire Core line in the PRD for free. This includes: Core Rules, Gamemastery Guide, Advanced Player's Guide, Ultimate Magic, Ultimate Combat, and both Bestiaries (soon to include Bestiary 3 and Advanced Race Guide).

Its their strategy to avoid internet piracy: give us everything for free so we literally have no reason to steal.

That said, the books are amazing quality and contain much more information (both fluff and crunch) than any comparable 3.5 volume. At the very least I'd suggest picking up a Core Rulebook.

Most of the non-core PF material is found on the OGC, which is updated by the fans.