I had a similar issue, wasn't able to find how to save to Dropbox from another app, different file extension but:
There is a "Save Document" in the second row. If you tap it the iCloud Drive dialog pops up and there is a "Locations" setting on the upper left. From there you can go More "Manage Locations" and enable Dropbox
Then Dropbox will always pop up instead of iCloud Drive when you select "Save Document"