Powerpoint files don't have an annotation layer, PDF does. That's also why e.g. Goodreader won't let you annotate Word files, because these apps are using the file formats as they were made, and as such need to save in a file format that support annotation. If you just want to view a PPT file, a dedicated note taking app isn't what you should be using anyways, which I'm guessing is the logic behind not having a view-only mode.