Thank you for your question.
Avenza Maps essentially detects the embedded referencing in the files which in this case, includes the white space around the maps. Although there is nothing there, the white space contains georeferencing so the extents of the georeferencing must be specified in the file prior to import. The only way to avoid this when scanning paper maps to digital would be to crop them in software like Geographic Imager for Adobe Photoshop. Unfortunately, in the app, there is nothing we can do at this time to prevent this another way.
Avenza Systems Inc.