How to repair a PDF

Hi,
I am having a problem trying to open a PDF file. Ever time I attempt to open, Adobe Reader loads, but displays a rather cryptic error message:

There was an error opening this document. Access denied.

It’s not a case of permissions, as I have administrative permission and can open all other PDFs on the server.

I cannot copy it or move it and it is being skipped over by the backup server.

All of this suggests to me that the file has become corrupted. Can anyone suggest a way of recovering this file without paying an outside vendor or using licensed software?