Here's how it works:
First, when you are using PACER and purchase a document, RECAP makes it easy for you to download a copy of the document to their repository.
Then, when you are searching PACER for a document and somebody else has already added that document to RECAP, you'll be notified that the document is already available for free. The people at RECAP (a project of the Center for Information Technology Policy at Princeton University) have pre-loaded documents from PublicResource.org, so there are already over a million documents available.
See their site for more information about copyright, privacy, fee waivers, and other issues regarding the use of RECAP.