Each document window must be closed individually. If you only have one file open, you can close it from the File menu and keep Word running.
Click the File tab.
Click the Close.
The file is closed, but Word is still running. You can still browse for a file to open, create a new document from a template, and access Word’s options.
If you have multiple Word documents open, clicking the Close button on the title bar will close each document individually.
If you have not saved the document since making changes, a dialog box will appear asking if you want to save the changes to the document. Click Save if you wish to save your changes, click Don't Save Save if you do not want to save your changes, and click Cancel if you do not want to close the document.