Yes there is a wall you hit with resolution when zooming in the app even if your PDF is very high DPI. The work around is to export your pdf map with a very large custom page size. The result is that you cannot zoom out fully to the entire extent in the app, but you still have the same capacity to zoom in (ie; at full zoom out, you are still zoomed in, relatively). This allows your map to have a large extent while still maintaining detail. I’ve been exporting maps with 2 X 2 meter page size.