file:///path/to/directory/some%20file%20name.pptx is the rough structure of the path and filename. Spaces are URL-encoded as %20, FYI—it’s not an Obsidian-specific thing. (There may be other characters that need to be URL encoded too.)
For an alternative approach, on the Mac/iOS, I use DEVONthink to maintain my files, mainly because DEVONthink provides a permanent unique URL for every item in a database. If you move a file you link using the above format, the link will break; DEVONthink or other related apps provide a sustainable alternative.