Cause: The document is in an earlier file format and has a .doc file extension.
Solution: Save the document in the .docx file format.
Important: The following procedure changes the file format of the document. If you plan to share the document with people who use an earlier version of Word, you might want to work in compatibility mode and leave the document in its current file format. This is because other people may be unable to edit parts of a document that you create by using new or improved features in Word 2011.
On the File menu, click Save As.
On the Format pop-up menu, click Word Document (.docx).
Make sure that the Maintain compatibility with Word 98-2004 check box is clear, and then click Save.