US Topo

US Topo maps and the maps of the Historical Topographic Map Collection (HTMC) can be downloaded free of charge, in PDF format,
Yes. The following applies to US Topo GeoPDF and the Historical Topographic Map Collection (HTMC) GeoPDF.
Traditional national mapping programs gathered data from primary sources, including direct field observation. Such maps were compiled, drawn, and edited by hand.
Yes. However, in most cases traditional lithographic printed maps are no longer available.
Yes, it is possible to convert GeoPDF to GeoTIFF with either commercial or open source software:
The original USGS 7.5-minute topographic map series (1945-1992) included feature classes that are not yet shown on US Topo maps.
There are different answers to this question for different products.
It is very rare today for traditional topographic maps to be used for driving navigation, so this issue is usually about GPS devices, smart phone applications, or commercial online maps.
Boundaries are an ongoing issue for the US Topo project due to the lack of national GIS datasets suitable for general-purpose, 1:24,000 scale maps. The earliest US Topo maps (2009-2010) showed no boundaries except the US national boundary.
No. US Topo maps are not legal documents, and the Public Land Survey System (PLSS) representation is not legally authoritative.