Go to tab "Bibliographic data"
Then at the bottom are the download options.
For future reference, one can often download from the DFG-Viewer by using the double arrows just to the left of the enlarge-reduce + and - the selecting the download icon.
For this work, however, the download icon did not "light up" so used the Bibliographic data tab.
Oh, yes, once you hit the download with a left click, it starts up; best is to right click and choose the location with "Save link as . . . "
Good luck. - R