We haven’t documented the file format (and probably don’t intend to), since our work on encoded types (post 1, post 2) means we’ll want to keep introducing new ways to encode data, which means external tools reading those files will need updates or break.
However, making a command-line utility like parquet-tools is something we’ve floated before – I think it would be great to have some way to do head or similar operations on our files.