The content of the game mechanics from Paizo is mostly Open Content. Anyone .. *anyone*.. can take the open contents and re-publish them, and charge for them, as long as they follow the terms of the Open Game License.
However... the images from the Core Rulebook, and the bestiaries, are NOT open content. Likewise, some names of Feats, Prestige Classes, and other things from other books are NOT open content and cannot be used without permission. Paizo provides a community use license, which provides for non-commercial use of SOME content.
All of which is why the best advice is.. if you are not working with material that is 100% your own, talk to a lawyer.
Note: The terms of the Open Game License aren't all that onerous.. you need to designate which (if any) of your content is open or not, you need to include a copy of the license, and you need to include the copyright sections of the OGL of the products you're referencing ("Section 15" of the license, in whole). Even so, talking to a lawyer first is wise.