The default file format in Word 2011 for Mac is *.docx. But for most of the time if you want the document to be saved as rich text format then you can change the default file format using Save Options. Listed below are the steps to set the default file format to .rtf in Word 2011.
Step 1: Launch Word 2011 for Mac.
Step 2: Click File menu and select Save As option from the menu list.
Step 3: In the Save As window, click the Options button.
Step 4: Click the “Save Word files as” drop down and select Rich Text Format (.rtf) from the list.
Step 5: Click OK button on the Save window to confirm and close the window. Then Cancel or Save As button in the Save As window.
Also See: Save Rich Text document as Word document using TextEdit on Mac