The copyright hasn't run out. It's been classed as
abandonware, which is still technically not legal. (Apparently copyrights last until 90 years after the death of the holder.)
But I'll leave that up to you to decide.
What I wanted to say was, I have never heard of .ppc files, but I know very little about Macs.
However there is a program called
ScummVM which designed solely to play LucasArts adventure games. Best of all it has versions for practically every OS out there. I've never used it myself, but you should be able to download the version you linked to and play it with ScummVM.