It contains precise positions and proper motions of 378,910 stars on the whole sky in the J2000/FK5 coordinate system.
It is designed to represent as closely as possible the IAU (1976) coordinate system on the sky, as defined by the FK5 star catalogue.
Thus, the PPM is an extension of the FK5 system to higher star densities and fainter magnitudes.
For fainter magnitudes, the supplemental list documentation states that, ... of the perhaps 20,000 stars between V=7.5 and V=8.5 only a few hundred are missing from PPM.
The data contained in the catalogue is downloadable from NASA (and other sites).