Total conversion mods often require you to extract the vanilla PK4 files to understand how id Software built their shaders, GUIs, or collision detection. Modders frequently download "dev packs" that contain unmodified PK4 assets.
Important legal note: You cannot legally download the original PK4 files for free from a random website. These files contain the copyrighted art, sound, and design of Doom 3. Distributing them without permission is piracy.
However, there are legal ways to obtain the contents of the PK4 files:
If you’re a budding modder, creating a PK4 file is simple: