The basic difference between PPT and PPTX files is that the first ones are the extensions for files in 2003 and earlier versions and the second are 2007 and beyond. In order to view the latter, the Free PPTX Viewer is an ideal choice. It is a very simple to use app that can be downloaded for free. There are no hidden charges or trial versions of the software and hence, it is considered to be one of the better known products.