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Ah, so the new version with the XML format has a "pptx" extension, whereas old PowerPoint saves as "ppt".

That explains a lot. The professor had Office pre-'07 installed. Your sister-in-law sent him a pptx file. Professor double-clicks on file. Windows pops up and says "I don't know what this file is". Professor is ignorant or lazy enough to not bother trying to open PowerPoint and selecting "File->Open". Professor gives up and says "it's not compatible".



That wouldn't have helped him, his pre-'07 copy of Powerpoint wouldn't have been open the file even via File->Open.
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