Custom MapsAuthor: Marko Teittinen
Current version: 1.8.4
See the app in Play Store | Custom Maps Users mailing list | Custom Maps in GitHub
Custom Maps is an Android app that allows converting almost any map image into a GPS map where you can see your own location. And after the map is created, Custom Maps can be used entirely without network connection far away from cell towers. Custom Maps can use PDF files, and jpg, png, and gif images as long as they contain an image of a map that is reasonably accurately drawn.
BTW, Custom Maps is entirely free, and it does not contain ads. I make no money with it. It is just an app I created for myself to make it easier to hiking in new places regardless of whether the area has data signal coverage or not.
Click the video thumbnail below to learn how to use Custom Maps 
The video was created by Bart Eisenberg.
Note: Many maps are copyrighted, and you cannot distribute them widely to other people. In most countries and most cases personal use is OK, but I am not a lawyer.
Some tips regarding some of the map sources listed above
Screenshots of webpages are not necessarily super useful since you could often just use the app provided by the mapping service. But they can be used for learning purposes, and in cases where the app from the mapping service does not work well offline, it might be useful to create a Custom Map from some screenshots.
If you plan to take a photo of a posted map at some location, remember that Custom Maps app needs a network connection when you are creating the GPS map. If you are going to a place that might have a PDF brochure available online, check that before going and create a map from it.
If you are scanning a paper map, it is best to avoid excessively high scanning resolution. Some scanners support over 1,000 dpi scanning resolution, but that will result in an image that is too large to load into a phone’s memory all at once. 300 dpi scans will show all the detail from the map you need to see.
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