As incorporated in the Seinfeld episode "The Barber", the Penske File is the project George is given to work on in a job he may or may not have. Unsure as to whether he has the job, he is equally unsure as to what work he has to do on this file.
Thus, to refer to something as the "Penske File" denotes being given a task, not knowing what to do with it, but going along with it as if you do. This can consequently result in a lot of 'doing nothing', while looking like you are working hard.
Quote from Seinfeld: "Yes, yes of course. The Penske file. Ho ho, can't wait to sink my teeth into that. Wow that Penske. Well we'll straighten him out."
Starting a new job: "Well they told me what kind of work I'll be doing.. Didn't really understand them though... Guess I'll be working on the Penske File"