On the Bluebeam website, under "Resources" > "Need an older version of Revu?" there is only v20 and older. This needs new options, like v21.6.
Currently the need for this is high, because v21.7 has a highly annoying "new feature" while drawing polylines, areas and contour and area measurement. Something goes visually wrong with the display of the temporary line segments. Installing v21.6 is a quick solution.