PowerPoint 2007 for Windows and PowerPoint 2008 for Mac will automatically save with a .pptx
extension which cannot be read by previous versions of Office. To avoid this problem, save down that is, save as an earlier version so that people using older versions of Office can open the file.
To save down in PowerPoint 2007 for Windows
Click the Microsoft Office button
, select Save As
, and select PowerPoint 97-2003 Presentation.
To save down in PowerPoint 2008 for Mac
Select File > Save As.
In the Format drop-down menu, choose PowerPoint 97-2004 Presentation (.ppt)