If this affects all documents, try clearing the Temp folder, which is typically:
C:\Users\%Username%\AppData\Local\Temp
See also: https://support.microsoft.com/en-us/kb/211632

If it's just a single document, or a few documents, it/they may acquired some of corruption. 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]