As to why it's reporting this, we don't know... What you are seeing is an error message that is created deep down in the application. Something went horribly wrong, so wrong that Reader was not able to display a more user-friendly error message.
What version of Reader are you using? On which operating system? When do you see this error message? Can you open other PDF files?
Karl Heinz Kremer
PDF Acrobatics Without a Net
PDF Software Development, Training and More...
http://www.khkonsulting.com