The file is a ZIP archive with a filename extension.pkpass containing a set of digitally signed files describing the digital pass. Multiple .pkpass files can be further combined into a single ZIP archive with an extension of .pkpasses in order to combine several digital passes for ease of distribution.
The file contents of a sample PKPASS file might look like this:
icon.png
icon@2x.png
pass.json
manifest.json
signature
en.lproj/
logo.png
logo@2x.png
pass.strings
ru.lproj/
logo.png
logo@2x.png
pass.strings
icon.png is the digital pass icon. logo.png is displayed at the top left of the rendered pass.
Localized resources - like images and strings - are stored in subfolders named like this: <language_identifier>-<region_identifier>.lproj
The file pass.json is a JSON formatted dictionary describing the digital pass.
manifest.json contains a JSON dictionary containing SHA-1 hashes for all files except the manifest itself and the signature.
signature contains a PKCS #7 signature of the manifest file thus effectively signing all files in the bundle.