You view a portion of a document on the screen through a

Document window is through which we view the portion of a document.

Explanation:

We have many portions like tabs, ruler, ribbon, etc in word. There is one another feature called Document Window in word where it represents the actual view of a document in GUI (Graphical User Interface) operating system.

Here the actual contents of the page are created and the user can also insert various shapes based on the need.  Any feature in the tabs can be included in the document window. You can set the zoom size of the document according to the need.