In Postlab, a Document is a file that is under version control. At this moment, FCPX libraries and Premiere Pro projects are supproted. A Document has a locking state, a status (E.g. "In progress...") and more.
Documents live inside Productions, which is a .postlab bundle on your hard drive.
There are two ways to back up your local Documents:
Back up your complete Postlab folder, including all production bundles.
Export Documents in Postlab: next to the version list, there's an option menu that allows you to export a single Document to your local hard drive.