Received several pdf files but cannot open most of them. Please help.

[vincent_su]

Here is the error message.

[Bernd Alheit]

Did you contact the person?

[JR Boulay]

In general, Acrobat's messages are true.
If it cannot read a file, it is because it does not comply with the specifications in PDF 1.7 format, aka ISO 32000 Standard.