Right, but if we find that we do have a file which is impossible to compress, we could just switch to a system where it does work. There may even be operating system variables you could change which would change the output of the program. The guy asked: "Does it have to run on any machine or is just one machine ok." But did not get an answer.

I agree, for any specific machine configuration there exist non-compressible files, but I don't think we are so restricted. *Edit, it might also be possible to include in your compression algorithm the logic: change the OS into a state where this file is compressible.