Pathfinder Roleplaying Game Reference Document
The Pathfinder Roleplaying Game Reference Document, also known as the Pathfinder RPG Reference Document or PRD, is a compilation of all the Open Game Content (OGC) contained within the core rules from the Pathfinder Roleplaying Game. It originally contained OGC taken from Pathfinder Roleplaying Game Core Rulebook, but after Pathfinder RPG Bestiary was published a second section of the PRD was added.
Unlike other parts of the Pathfinder RPG core rules, the Pathfinder RPG Reference Document is free and can be downloaded from the Paizo website.
Reasons for creating the PRD
With all Pathfinder products being released under the Open Game Licence (OGL), Paizo Publishing have already opened up a lot of their rules to third parties. But the Pathfinder RPG Reference Document creates two advantages over Paizo's obligations under the OGL.
Ease of use
One of the restrictions of the OGL is that Product Identity (PI) must not be used by anyone reusing content published under the OGL. Reusing PI, even accidentally, is a breach of the conditions of the OGL and can cause legal problems for anyone who does this. With the PI clause broken, a product looses the protection of the Open Game Licence and may need to be recalled or destroyed.
The Pathfinder Reference Document simplifies OGC reuse, because it consists entirely of Open Game Content and has none of Paizo's Product Identity within it. This means that, like the 3.0 and 3.5 versions of the System Resource Document from Wizards of the Coast, the PRD is able to function as an easy to reuse game engine.
As long as a copy of the Open Game Licence is attached to their products, publishers and private individuals can use the Pathfinder Reference Document for any purpose they wish. It is even legal to publish the entire content of the PRD, in the same form, or in a modified form.
Encouraging further support for the third edition rules
With Wizards of the Coast abandoning use of both the System Reference Document and the Open Game Licence, some publishers turned away from supporting third edition and moved towards totally different rules. Many of these rules systems were closed systems (meaning that compatible products could not be sold by different companies). Some systems were open, but where published under different licences, that were incompatible with existing third edition material.
In contrast to this, the PRD picks up from where the last version of the SRD left off. Publishers who wish to, can use the new reference document to make products that are compatible with the new Pathfinder core rules.
The ease of use, mentioned above, also combines with compatibility with other OGL documents to allow people to mix and match rules from the 3.0 SRD, the 3.5 SRD and the PRD. It is even possible for the PRD to be combined with non-fantasy reference documents like the Modern SRD.
At the moment, it is not known if other OGC from unpublished core rulebooks (such as GameMastery Guide, Advanced Player's Guide and Bestiary II) will be added to the Pathfinder RPG Reference Document.
However, in the past errata from the published products has been added, making the PRD more up to date than printed rulebooks, so further updates to the document are expected.
Where to find the PRD
Paizo make the Pathfinder Roleplaying Game Reference Document available on their website. It is in a text form, in multiple pages.
The d20pfsrd.com website is a wiki that aims to create a hyperlinked version of the Pathfinder RPG Reference Document.