Marc Miller (Software Engineer) has posted a very important information in The Official Gmail Blog about PowerPoint and TIFF file viewing. It is a good news for all gmail users that their previously offered PDF viewer will now be supporting TIFF as well as PPT files online.
This viewer has much more features than the slideshow as you will be able to Zoom in or Zoom Out the document. Besides this the copy and pasting of document has been made possible in it. The printing of document is also easy and has option of "open as ready to print".