ProPack doesn't use compressed files.
P5 can compress everything except the geometries and thumbnails.
P6 can additionally compress geometries (OBJ format).

Both P5 and P6 come with scripts to compress and uncompress files (these are based on the standard Python gzip module).
Any compression utility that understands gzip format can extract files.

A lot of 3rd party Poser utilities won't work with compressed files.