It is ok to generate presentation file using existing pot file,
BUT, when using potx file, the same code will produce EXCEPTION!!
As the following code depicted:
(Note: the potx file is produced by just merely saved-as from the pot file)
ISlide slide;
Presentation ppt=new Presentation();
Presentation masterppt=new Presentation();
// the following can produce result successfully.
masterppt.LoadFromFile("IT.pot",FileFormat.Auto);
slide=masterppt.Slides[0];
ppt.Slides.Insert(0,slide);
ppt.SaveToFile("result-from-POT.pptx",FileFormat.Auto);
// the following will Exception!! ("Object reference not set....")
masterppt.LoadFromFile("IT.potx",FileFormat.Auto);
slide=masterppt.Slides[0];
ppt.Slides.Insert(0,slide);
ppt.SaveToFile("result-from-POTX.pptx",FileFormat.Auto);