If I recall correctly, the type of Steam cloud that I use (legacy version) does not sync between Mac and PC. Sorry about that.
Yes, you can definitely do it manually.
On Mac the save files are in: ~Library/Application Support/Big Pharma/Saves/
On Windows the save files are in: C:\Users<username>\Documents\My Games\Big Pharma\Saves
You might want to temporarily disable Steam Cloud saves when you copy the saves in. Probably not necessary though.
I’m sorry about the inconvenience, I know it’ll be a pain to switch back and forth. For my next game, I’m definitely going to implement Steam’s new cloud save system which is cross-platform compatible.