From QWiki
For the polish Quake player, see Pak1.

Pak1.pak is the registered Quake data file that contains the episode 2-4 levels and the deathmatch levels (dm1-6) along with the sounds and models required to play these. It also contains the bonus level e1m8 that is entered from a secret portal on e1m4. Pak1.pak resides in ./Quake/id1/.