You would think that Microsoft would allow the content provider to control how a PowerPoint Presentation is displayed to the user from the browser.  However, not possible, it will open in whatever mode has been determined by the client side registry setting: HKEY_CLASSES_ROOT\PowerPoint.Show.8

When you view a Hypertext Markup Language (HTML) document in a Web browser and click a hyperlink that references a PowerPoint presentation, the presentation opens in slide show mode in the browser window. This article explains how to change this default so that you can open the presentation in a separate window or open the presentation for editing.