How can I copy a saved game file from the PC to a PS3 and get it to load?

Save it to a memory stick, enter said memory stick to ps3 copy and use.
Maybe where you got the file from also has a read me file, this file should include transfer options.

but I’m not sure if it will work on PS3. From what I know the files are incompatible (unless the file you downloaded was a PS3 file to start with)

If the file originally was saved on a Memory Stick, then you can’t – it uses Magic Gate technology to prevent that.