You can make a copy of a document by first editing the document by clicking on the blue Edit command that appears to the left of the document title. Once you are in the editor, choose the "Copy Document" button:

The editor instantly makes a copy of the document in the same folder and puts you in the folder view. The new copy of the document will have a new name beginning with 'Copy of' and ending with the date and time the copy was made.

For instance, if your document is titled 'Quarterly report' it would originally appear in your folder view as this:

Once you made a copy of it, the copy would appear with this name:

Note that a great way to archive a version of your document is to give the copy command, then 'hide' the copy so it doesn't clutter your folder.