A quick google search found the following:
.PK3: Quake III PAK file.
Renamed .ZIP file. It is strongly recommended that .PK3 files installed with commercial games not be modified or extracted and re-compressed. All known Quake III Engine games check the exact size (and maybe other feathers) of a .PAK file before using it and any modification would irreversibly corrupt the file; the file would still be viewable, but the game's executable would no longer recognize it. However, if viewed with a hands-off aproach one should find music, textures, text and the like easily extracted and viewed without harm to the functionality of the games that require them.
.PK3: Quake III PAK file.
Renamed .ZIP file. It is strongly recommended that .PK3 files installed with commercial games not be modified or extracted and re-compressed. All known Quake III Engine games check the exact size (and maybe other feathers) of a .PAK file before using it and any modification would irreversibly corrupt the file; the file would still be viewable, but the game's executable would no longer recognize it. However, if viewed with a hands-off aproach one should find music, textures, text and the like easily extracted and viewed without harm to the functionality of the games that require them.