When you have opened more than one word document, you can close the document by clicking the Close button available on the top right corner.
Another alternative way is by navigating to the Windows taskbar and clicking the close button on the respective document thumbnail.
But if you have opened only one Word document then clicking the close button on the top right hand corner would close the application itself. One solution for this problem is by adding close button to the quick access toolbar.
Step 1: Click the Customize Quick Access Toolbar and select More Commands from the list.
Step 2: Select All Commands in Choose commands from drop down list.
Step 3: From All Commands list, select Close option and add it to the Customize Quick Access Toolbar.
Step 4: Click OK button to confirm and save the changes.
Now you can quickly close only the document by clicking the Close option available as part of Quick Access Toolbar (QAT).