Can you transfer game files on Mac to a PC?
So here is my situation:
I downloaded "Slime Rancher" on Steam on a Mac computer. When I try to play it on my new PC, it is very, very slow. My suspicion is that it is because I downloaded a Mac version, and I need to buy the PC version for it to work.
If I buy the game again, I would assume it wouldn't save my progress, so my question is: Can I transfer the game files between the two?
(Also keep in mind I don't really know anything about Steam, so it could be a completely different problem.)
I downloaded "Slime Rancher" on Steam on a Mac computer. When I try to play it on my new PC, it is very, very slow. My suspicion is that it is because I downloaded a Mac version, and I need to buy the PC version for it to work.
Your suspicion is incorrect.
First: When you purchase a game on Steam, you automatically get access to all available versions of the game. Windows, Mac, and Linux versions of games are not sold separately -- if a game is available for multiple platforms, you will get access to all versions, and you will always download the version for whatever operating system you are running. (There is no way to download a different version, even on purpose.)
Second: Mac games will not run at all on a Windows system. Period. The fact that the game is running is proof positive that you got the right one.
Many Steam games, including Slime Rancher, support Steam Cloud. This will allow you to transfer saved games between computers (which will usually -- but not always! -- include transferring games between Windows and Mac versions). Steam Cloud is automatic; you do not need to take any extra steps to transfer your saved game.
That being said, if the game is running poorly on your PC, there is probably something wrong with the way you have your computer configured. Without any hardware details, though, it's difficult to say what might be wrong.
If an app you buy on Steam supports Steam Play then you automatically get access to the different platform-specific versions of it if/when they exist. This is easily noticeable on a game's store page when there are icons for multiple different platforms above the "Add to Cart" button. One special example where you do not get the different versions is Call of Duty: Black Ops which does have Mac and Windows versions...but they are totally different purchases =| Thankfully Slime Rancher seems to support Steam Play.
And while it is possible to download a version of a game that's built for a different platform it is not simple and would require you to do some magic with Steam as I went on and on about here and would not happen automatically. Also, Mac versions of games cannot run on Windows without using a VM just like Windows versions cannot run on Mac without using a VM (at least, not easily; Wine does make it possible, but requires work).
Finally, the game does support Steam Cloud and so as long as you have that enabled for the game itself & for your Steam client (and have internet) it will synchronize with the cloud and with your different machines. I don't know personally if it supports cross-platform saves, but based on it's Steam Cloud configuration as listed on SteamDB I would guess that it does, even though there is no definitive data that says so.
As duskwuff said though, if the game is running badly that's mostly likely your system or settings in the game [as long as your machine meets the minimum system requirements]. Try lowering the settings to their lowest possible values and then test the game again, and then raise the resolution until it starts running worse; if you get to maximum you can try turning on some other things like forms of anti-aliasing (AA), but you may need to lower the resolution for it to run smoothly with unnecessary settings turned on.
If you have other apps running in the background or have the Steam Overlay enabled this can also effect a machine's ability to run a game pretty easily. Try quitting all Cloud service and other unnecessary background apps before running games. You can also try disabling the Steam overlay for the Steam client (or for that game if you want to keep it on for the client). Overall these likely won't give you much of a performance boost, but sometimes every little bit more of power can help a lot.