As far as I know, BackCountryNavigator does not support Garmin maps. The only Android app that I know of that does support Garmin maps is Orux. The app is free but the rendering is not brilliant quality.
If only the PDFs were GeoReferenced, then it would be easy.
The workflow would be:
1. Download a PDF 2. Identify coordinate points for georeferencing (see below) 3. Georeference the PDF by matching points to coordinates 4. Crop the collar to just leave the map detail 5. Cut the map into 1Mpixel pieces to make a KMZ for Google Earth and/or a Garmin Custom Map.