Keynote document select:
File > Export > PPT...
or better (to avoid most problems with PowerPoint format):
File > Export > PDF...
This last format will work perfectly on any Unix and any Windows.
Warning: test these formats on the target system before a serious presentation.
With both formats, video, animation, transitions and presenter notes will be lost.
If your projector is of the VGA era (see https://www.google.fr/search?q=vga&source=lnms&tbm=isch) then I would advise you to:
- Stay with Keynote (this software is still many years ahead of many others and mostly of PowerPoint).
- Buy a Mini DisplayPort to VGA adapter.