tjm1706:

Showing an offline map using 1 Mapsforge Osm Map file is only possible in a HTML5 webapp using commercial tools like Mapbox. There are simply no free tools to do so, as you can read in this post.

Is it possible to show an offline Mapsforge Osm Map in an Progressive Web App? That would really be great.

Why an offline Mapsforge file? It is a free and good OSM map. It is available as 1 file per (great) area, so not a huge bunch of small files in a x/y/z folder.