Both .ppsx and .ppt files are presentations made in Microsoft PowerPoint, so this is the software for most convenient way how to handle their conversion.
Open the .ppsx file in the PowerPoint 2007 and later ► Click on File menu ► Save As ► File Type PowerPoint 97-2003 (.ppt) presentation ► fill the File name ► Save button.
It is likely that ppsx to ppt conversion might also be doable in other presentation tools that have PowerPoint format support, but using the original program is always the best for the sake of compatibility.Open PPSX file Open PPT file
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