I've purchased CE for the Mac, but I have a friend who uses a PC system. How would (If possible.) I go about downloading a PC version of the game?
If you log onto Steam from a specific platform, Steam just downloads a copy of the game for that platform. Log on on a Windows machine, you get a Windows copy etc. That said, CE is a single-use license! Your friend should buy a copy on their own and support independent gaming.
