USGS offers both the most current topographic maps (called "US Topo") and the Historic Topographic maps for free web download from the USGS Store. Discover and download from store.usgs.gov (select "Map Viewer and Downloader" at the upper right). While the US Topo product contains current aerial imagery for viewing, please note that hiking trails are not yet available for many states in this product; in many cases you may need to also get the historic map if you are looking for trail information.
Each map is delivered in PDF format with geospatial extensions (GeoPDF®). When viewed on a GPS enabled device, like a smartphone or tablet, the geospatial extensions allow users to see their location on the product, as well as add waypoints, tracks/trails, or other features allowed by the application. While the GeoPDF is layered, with the ability to turn on/off various layers (such as the ortho imagery, for instance) using a common PDF reader in a desktop environment, the GeoPDF will open on a mobile device with the default layers turned on. Currently, there are not any GeoPDF readers in iOS or Android which allow user customization of the GeoPDF file via the PDF’s various layers.
Avenza's PDF Maps Application
For both Android and iOS devices, Avenza’s "PDF Maps" app is available to read GeoPDF files like the USGS US Topo and Historic Topo Maps. Avenza’s application can be found here:
Importing US Topo or Historic Topo
Once the US Topo or Historic Topo is discovered and downloaded from the USGS Mobile Client (http://viewer.nationalmap.gov/apps/mobile_client/#/home):
Disclaimer: Any use of trade, firm or product names does not imply endorsement by the U.S. Government. No warranty, expressed or implied, is made by the USGS or the U.S. Government as to the accuracy and functioning of the commercial software programs cited in this Technical Announcement, and the U.S. Government shall not be held liable for improper or incorrect use of The National Map topographic data from USGS.