The Save commands default file format must be configured.
When users create new PowerPoint files, PowerPoint 2013 saves them in the new *.pptx format. Ensure this setting is enabled to specify that all new files are created in PowerPoint 2013. If a new file is created in an earlier format, some users may not be able to open or use the file, or they may choose a format this is less secure than the PowerPoint 2013 format. Users can still select a specific format when they save files, but they cannot change default of this setting from the PowerPoint Options dialog box. This enforced user behavior ensures any change to the file format requires additional deliberate user interaction.
Verify the policy value for User Configuration -> Administrative Templates -> Microsoft PowerPoint 2013 -> PowerPoint Options -> Save "default file format" is set to "Enabled PowerPoint Presentation (*.pptx)".
Procedure: Use the Windows Registry Editor to navigate to the following key: