Retrieving Steam Game progress from Mac to new PC

This is not always possible because as others have said the Mac, Linux, and Windows versions of a game save are often of a slightly-different format. This has also already been asked and answered before here

So, BL2 is a special case which does allow this, but must be done manually.

Edit: Manual instructions (since I couldn't edit the comment below fast enough -- sorry!)

The second answer on the question I linked to above covers that, but I'll summarize from their directions:

• Find the save data

Mac - Users/MYNAME/Library/Application Support/Borderlands 2/WillowGame/SaveData/SteamID64/

Windows - C:\Users\MYNAME\Documents\My Games\Borderlands 2\WillowGame\SaveData\SteamID64\

Copy the files from the one to the other to "synch" them.

You could setup "selective synch" with a Cloud service to make this automatic. I used to use Dropbox and it can do this, but I don't suggest Dropbox though, especially on Mac (Google Mac Dropbox exploit).