Please read the PowerPoint Viewer download page which includes this statement: Until Red Hat creates new metrically equivalent fonts ( as it did for the older Microsoft fonts), it's easy to install these new Microsoft fonts without Vista or Office.
pptx, chances are you need these new fonts, but, Linux, and Windows XP users need not be left out in the cold. When passing documents between systems, it's necessary to have the same: otherwise, documents look and layout may suffer. They replace the classics Times New Roman, Arial, Courier New, etc. Windows Vista and Microsoft Office 2007 introduce the new ClearType Collection typefaces: Calibri, Cambria, Candara, Consolas, Constantia, and Corbel.