Hi @StewartHollands,
I'll say that, unfortunately, what you're looking to do isn't really possible within Canvas itself. You can choose to make the file only visible to students with a link, and then embed it on a page in Canvas, but it would still be downloadable by students. There may be some third-party websites which could turn the PowerPoint into more of a web format to embed in Canvas without download, but I don't personally know of any specific one to recommend to try. On the web in general, I'd say you'll almost need to expect people will be able to download things, aside from some very specific use cases like streaming commercial video sites (Netflix, etc).
Sorry that this probably isn't the answer you wanted to hear, but hopefully the info helps a bit.
-Chris