Well, the spec says "Pass files are stored on disk as a zipped package with the pkpass file extension."

Without ever seeing a passbook file, I take that to mean it is just an archive (zipfile, tgz, lza,...) that contain a bunch of images and some identification data.
It might be openable with standard command line tools; for example strip extra headers off with dd, run the file thru unzip, sort through the results, display the flight details, codes & barcode...