Next to the menu list, you can see Recent documents list.
This list offers a quick way for you to access files recently used.

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As you open more documents, the list of recently used documents grows. The most recent ones appear on the top and you can open the same documents directly from the list.
Recent documents list also allows you to pin documents onto the list regardless of their last usage.
Once pinned, a document will always appear there.
To pin a document, just Click on the Pushpin icon to the right of the file name on the Office menu.
To unpin the same document, click on the Pushpin icon again.

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Click on the Pushpin icon.