It references an episode of Seinfeld where George showed up to a job that he wasn't sure he was hired for. He moves into the small office and was told to work on the Penski File. He was not sure what he was supposed to do so he did nothing. It is used when you are at work with nothing to do.
Jim: Hey Tom! What are you doing?
Tom: I'm just here at my desk working on the Penski File.