A GeoPDF is a georeferenced PDF file, meaning that it is a Portable Document Format (PDF) file with added information that relates the image to coordinates on a map. GeoPDF files can be used as plain PDF files but have the added capability of some limited mapping functions (turn any map layers on and off, obtain XY coordinates for a location, measure length, and more).
The USGS currently produces topographic maps using an Open Source GeoPDF format. However, most digital USGS topographic maps published before June 2017 are GeoPDF® files that were produced using software from Terrago Technologies. Those products are GeoMark enabled, so users can use the Terrago toolbar plugin to perform added functions.