Technically, what you are able to reproduce from any OGL work is everything that the OGL statement in that work says you can.
Now, it's very likely that the PRD has most of that from most of the Pathfinder books, and that it's a pretty good arbiter of how to interpret the OGL statement when it's not clear. However, that's not guaranteed, and if you're getting set to do hundreds of man-hours of work yourself, you should first read and understand that statement because that's the real "letter of the law." The PRD may not have everything you're entitled to reproduce, for example.
Copying from the d20PFSRD is similarly not guaranteed. If Paizo makes a mistake on their PRD - well, they're certainly not going to sue themselves... d20PFSRD pulls from many locations and makes improvements/alterations. It's "probably safe" but again, the real guideline isn't what someone else has decided is open content, it's what the OGL states is open content.
You are looking for statement of Product Identity (can not copy) and Open Content (can copy). It will read like this (this one cut and pasted from the Carrion Crown AP):
Product Identity: The following items are hereby identified as Product Identity, as defined in the Open Game License version 1.0a, Section 1(e), and are not Open Content: All trademarks, registered trademarks, proper names (characters, deities, etc.), dialogue, plots, storylines, locations, characters, artwork, and trade dress. (Elements that have previously been designated as Open Game Content or are in the public domain are not included in this declaration.)
Open Content: Except for material designated as Product Identity (see above), the game mechanics of this Paizo Publishing game product are Open Game Content, as defined in the Open Game License version 1.0a Section 1(d). No portion of this work other than the material designated as Open Game Content may be reproduced in any form without written permission.