3 is the latest that works on a 68k, and 5 on a PPC running Classic (though I'm not sure about the maximum version on a PPC running less than 9.1, as I enver use anything less than that on a PPC). Usually 3 gives errors when you open the document as it reports to it that it's designed for later versions, but it will usually display largely as normal, although I have tried this on my 520 and it is pretty slow, especially with more complex documents that contain diagrams, images etc.