Cannot open PDF Files after recovery

Hi, a friend of mine deleted accidentally from an external hard drive a very important folder and asked for help.

He told me immediately after the accident, I told him to unplug the harddrive without deleting or adding any file.

Then I brought the hdd to my home and plugged to my pc.

I used recuva which found the missing files (150 files , 150 MB total more or less). I restored them but none of them can be opened. Files are all .pdf

If I open with notepad ++ I can only see "nul" non printable character.

Any advice?

Thanks

If you open the files with Notepad, or look at the header in Recuva, the file should start with %PDF-1.4 or 1.5. If there is no %PDF at the start then it's most likely that the data isn't a pdf and won't contain your data. I assume the disk is NTFS?