From the announcement:
Welcome to the Pathfinder Playtest! Along with all the other members of the Design Team and the rest of the Paizo staff, we'd like to thank you for your time, dedication, and enthusiasm. Without you, we could not hope to make the best possible version of the Pathfinder Roleplaying Game. Your feedback, thoughts, concerns, and criticism are vital to our process—we literally can't do it without you!
So, let's talk about all the ways you can help us playtest the newest version of Pathfinder! Starting on Thursday, August 2, make sure to grab the free PDF of the Pathfinder Playtest Rulebook and make your first character. If you are planning on being your group's Game Master, download the free adventure Doomsday Dawn and start reading through Part 1 of the adventure. Make sure to also grab the free Pathfinder Playtest Bestiary PDF, since it contains all of the adversaries you will need to run the adventure.
Once you've gathered a group together, plan on playing regularly over the next few months as we make our way through all seven parts of the adventure. Doomsday Dawn is broken up into seven distinct chapters, each of which tests different levels and different aspects of the game. Make sure to set aside some time to give us your feedback as well. We are including surveys this time around to collect your thoughts and help you get your feedback to us as quickly and efficiently for you as possible, so taking part in the playtest could not be easier!