You can't really convert it back, but you have some options:
1) Save the PDF as a jpg or png file, then batch import them into PowerPoint
2) Use the snapshot tool in Adobe to copy slides in your PDF and paste them
into each slide in PowerPoint.
Steve Rindsberg ( I think) once said "converting a ppt to a pdf is like
taking steak and making meat pie, getting back to a ppt is like trying to
make the meat pie back into steak!"