As far as we know, the only software that can reliably open and otherwise work with Keynote presentations (.key) is of Keynote, now a standalone app that was before marketed as part of iWorks. Keynote has support for PowerPoint and can easily export .key and .keynote files to PowerPoint presentations (.ppt, .pps etc.), which icludes key to ppt conversion.
For Windows users this can be a little troubles as there is no Windows version of Keynote, nevertheless some latest versions of LibreOffice should be able to open at least some versions of Keynote presentations in the Impress program. Last but least, If you have Apple ID, you can try the online version of iWorks, that might offer tools to convert key to ppt.Open KEY file Open PPT file
Presentation software for iOS operating system, developed by Apple
The most popular presentation software for Mac
Powerful multimedia presentation software tool from LibreOffice suite