I don't really see much special here, aside from the fact that it can use geoPDF files. The USGS distributes its historical topo maps in geoPDF format, but they are just scans of individual quads, including the border, title block etc. So each of those would only cover about 6 miles x 7 miles, which doesn't seem especially useful to me.
Did not dig very deep, but it says it can also import JPG files if you have the associated projection and world files (something an average person with consumer type software probably won't have). It can also use geoTIFF files, which are used by professional GIS software but not so much by consumer software. The USGS used to offer downloadable geoTIFF format maps, but has moved away from that in recent years.
Anyway, I have software that could be used to make maps in these formats, but most other people probably don't. If you want compatibility with my maps, I'd say you are better off with the two dozen plus apps that I support: https://boydsmaps.com/mobile-atlas-creator/
Now, perhaps there is something else special about that app that would make it appeal to someone? Did not look very closely. FWIW, I am hoping to roll out a mobile version of my site this summer so you can access my maps on your phone. More on this later.