Since PowerPoint 2007 and beyond, the newer PPTX version has been widely used for saving PowerPoint presentations.
PPTX is not Supported by Older PowerPoint Versions #
You can’t save a PowerPoint file to PPTX in older versions like PowerPoint 2003. PPTX is supported by newer versions like PowerPoint 2007,2010, 2013, 2016, 2019, and Office 365.
The X in PPTX is for XML Support #
The newer file version of PowerPoint i.e. PPTX has an X added to its file extension which stands for XML. This is because this version comes with opensource XML support which makes it more compatible with different types of programs. So, if you want to use your PowerPoint file with other applications, you might just opt for PPTX.