pp files are encrypted or compressed. The encryption key is different for each game (sometimes Illusion changed the key for the game after a release of a demo).
Modding pp files requires decryption in order to modify any subfile in them. Usually there is a decrypter for each game. These decrypters are also able to encrypt and pack pp files. Additionally the decryption/encryption is integrated into SB3Utility, a tool which allows exporting and importing 3d data and much more.