Left-4-dead-2-3dm -

: Use a tool like 7-Zip or WinRAR to extract the multi-part archive files into a single game folder.

If you are looking to "prepare" the game for custom content or modding, the process is different: left-4-dead-2-3dm

: The standard format for Left 4 Dead 2 mods is .vpk . : Use a tool like 7-Zip or WinRAR