Powerpoint viewer comes pre-installed as part of Windows. It's a background program that activates when you try to view a PowerPoint document, but don't have PowerPoint installed. Basically, it let's you see the PowerPoint presentation, but you can not edit or change it in any way.
Well you have to download the powerpoint presentation, but not the actual powerpoint program.
Yeah, not sure how you're supposed to view it online. Files like that generally have to be downloaded before being opened. Your professor might not be all that good LOL Good luck :)