Cannot fix Adobe Reader error

I received many PDF attachments on my email from a friend who wanted me to go through students’ projects. When I tried to open them I would get an error from each that I have checked so far as shown below.
Suggestions?

Adobe Reader could not open ‘filename.pdf’ because it is either not a supported file type or because the file has been damaged (for example, it was sent as an email attachment and wasn’t correctly decoded).
