Wow, I didn't realize so many people actually think it's not coming to PC. It 100% is. RDR1 was a mess towards the end of development and just getting it released on consoles as it was didn't come easy for them. They were also hard at work on GTAV and as soon as RDR production wrapped they began shifting all of the company's resources to GTA. I think with RDR1 being as difficult to work on as it was, combined with how big of a project GTAV became, they simply didn't feel like they could justify the resources at the time.
Now they've seen what a monster GTAV is on PC, with a very sustainable ongoing income system on all of the platforms with their micro-transactions, they aren't going to pass it up with RDR2 especially since they've had time to engineer it this way from the start with PC in mind. They learned a lot about what works with GTAV and you can bet with certainty this game will follow that exact same formula to a T. It would be silly not to, so expect the PC release in about a year.