Seems like you can get Arma 1 for free on Steam atm. Don't know how long it'll last but get it while you can I suppose

Use my shady link: steam://run/65790/

or if you don't trust it, go the Steam store page of ARMA: Cold War Assault and click "install game". You don't have to install it but it will be added to your account for free.

Cheers!