In that case, either the document is empty or corrupt, you have an errant third-party addin or the problem relates to a faulty Windows installation.

Try starting Word in Safe mode (hold down the Ctrl key while starting Word). If you can save as PDFs in Safe mode, an errant third-party addin is the likely culprit.

Corrupt documents can often be 'repaired' by inserting a new, empty, paragraph at the very end, copying everything except that new paragraph to a new document based on the same template (headers & footers may need to be copied separately), closing the old document and saving the new one over it.

__________________
Cheers,
Paul Edstein
[Fmr MS MVP - Word]