Copy both the ACF file and the game folder to the appropriate Steam folder on a different PC and when you next start Steam, you'll find the new game listed.

Verify the integrity of the game files on Steam, Epic, GOG Galaxy, Ubisoft Connect, Battle.net, Origin, Rockstar Games Launcher by following this tutorial.

Repeat this process on all your computers, so that whenever you start up the game—no matter what PC you're on—your game gets redirected to the files in your Dropbox folder.