Roy Rogers, posing as The Carson City Kid, is seeking vengeance on Morgan Reynolds (Bob Steele), the man who killed his brother. To find Reynolds in the gold towns, he systematically stops stagecoaches and goes through the mail, hoping to find letters addressed to Reynolds and thusly learn his whereabouts. Thus "The Kid" earns the reputation of a stagecoach robber, although he never takes anything, and the reputation is enhanced by the fact that he travels with Laramie (Francis McDonald), a notorious half-breed outlaw.
You can stream Carson City Kid (1940) free and legally on OldReel, via the Internet Archive.
Yes — it is in the public domain and can be watched and shared freely.