The slideshow viewer shows Powerpoint presentations within the browser and is accessed via a “view as slideshow” option next to attached ppt files.
Google CEO Eric Schmidt formally announced in April that Google was developing an online presentation creation tool to compete directly with Microsoft. The new presentation viewer is the first released feature of the fuller Powerpoint like package.
ppt files are viewed via following the text link
navigation via backwards/ forwards button directly above the presentation, or via direct links to each page on the right
(via Google Blogoscoped)