You can include links to files (actually, not just documents) within the details of Events, Tasks and Meetings easily.

In the Storage area, find your file and click on its link.

From the popup box, copy the Share Link

You can then include a link to that document (or spreadsheet, or image...whatever!) in the description field of an Email or even Event.

To add the link to a description, highlight some text to use as a link and click the link button (see arrow below) in the toolbar and then paste your Share Link URL into the URL field. 

Your link will appear in the text.

Note: You can also add a link to a Task's or even a Meeting's description, but you need to add the link with HTML.

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