When I leave the asphalt roads, I really like knowing where I am, where I've been and where I'm going. GPS units to do an amazingly good job of on-road navigation, but I can't rely on them for off-asphalt travelling. They just don't have those roads in their map data base. I've managed to collect maps from government sources like the US Dept of Agriculture, US Forest Service, the BLM and others, and from tracks that I have run and recorded. I have keyed in many KMZ routes myself from scratch using Google Earth. The KMZ files can be loaded into many GPS units and tablets or phones that run an appropriate mapping application. Unforutnately, the PDF files are not geotagged for loading into a mapping application. They are only a digital copy of the paper maps.
You can find and download KMZ files for many of the listed trails and PDF maps files for most of the forests, parks, monuments, California State Parks and OHVs, most of which are Motor Vehicle Use Maps (MVUM). Some areas only have visitor maps and a few have maps created by other individuals.