Each time you open a document, Word places a push pin icon to the right of the document. Click the push pin and Word places the document at the top of the list and changes the push pin to a pin icon and draws a dividing line so you know to look above the line for pinned documents.
When you no longer need to have the document pinned, click the pin icon and Word moves the document back into the general list. The document rotates off of the list in due course.
The push pin icon is a common icon that you see in many programs. So when you see them, click them and see what gets pinned and where.