Open the original Powerpoint file, Go to File>Print and select Acrobat Distiller as your printer. Put a checkmark in the 'print to file' box, and click Print. This will create a postscript file (which may have .prn listed as the file type instead of .ps - I think it's a Microsft thing). Launch Distiller, go to File>Open, change the file types to 'show all files' and you should see your .prn file. Select it and click OK. This should generate your PDF.