Recovered file won’t open in Powerpoint

I accidentally shift-deleted a Powerpoint presentation (misread the number) and then recovered it using a few different programs (Undelete-plus, PCI File Recovery, Data Recovery). However, Powerpoint was unable to open the files, and gave the message “Powerpoint can’t open the type of file respresented by …” The selected file does not appear to be a valid Microsoft PowerPoint file.” In all cases, the programs indicated that the deleted file was in excellent condition and produced the same sized recovered file. Is there a way to modify the header info to allow Powerpoint to open it?