Do you have a PDF reader installed?
///Peter
Yes I do... that is why I am a little bit confused. Plus it never
happened before.
If it's Windows, something in the Registry has gotten out of whack and
no longer knows what to do with a filename ending in ".pdf". I think you
can force it back in My Computer, under View|File Types|Add New Type or
something, and link "Open" for PDF to wherever your acroread.exe lives.
///Peter
Thank you very much I will have a look. Hope it will work.
I'm almost sure you are running Windows and using TeXnicCenter to edit
your documents. It has to do with some incompatibilities with newer
versions of Acrobat Reader.
This might be useful:
http://www.latex-community.org/viewtopic.php?f=31&t=14
Best,
Juan