I tried to download the app and make it running. I think you can just download the widget through the toolbox within Mendix Desktop Moderler you select GroupsDocs Viewer.
You have to have a GroupsDoc account, because when you add the widget it will ask you for the log in of your GroupsDoc. After filling it in I think it should work after running it.
Hope you will be able to let it run. If it does not work look at the documentation of the widget as their might be some things explained for letting you run it. Normally they have on github a test version ready, however I looked and it seems there is no github present anymore.