US Topo

GeoMark is TerraGo's spatial data extension to PDF markup.
US Topo production follows the U.S. Department of Agriculture National Agricultural Imagery Program (NAIP).
Except for the following three cases, these maps have no copyright: Most maps contain commercially licensed road data for all areas outside National Forests (roads inside National Forests are public domain data prov
GeoPDF is a TerraGo Technologies published extension to the Adobe PDF file format. GeoPDF is not a separate format; any GeoPDF conforms to the base PDF specification, and GeoPDF files can be used as plain PDF files. More information:
US Topo project objectives are continuous product improvement, a rapid refresh cycle, and automated production using currently best available data. It is therefore unlikely there will ever be a final and ultimate product. 
Unlike traditional topographic maps, US Topo maps do not carry an accuracy statement in the map collar.
Maybe, but probably not soon.
Users can interact with the map on a computer screen without GIS software or expertise. For example, ground coordinates in a variety of reference systems can be displayed, and ground distances can be measured.
See the US Topo Map and Historical Topographic Map Users Guide.
Most GIS vendors do not yet provide geospatial PDF import capabilities. US Topo maps are derived from GIS data sets and are formatted as PDF for the benefit of non-specialist users.