The best mobile app for viewing maps created in QGIS that I have found is QField.
QField allows viewing of .QGZ project files saved in QGIS on mobile devices.
Currently QField is Android only, though an iOS version is apparently in the works.
Simply save the .QGZ/.QGS file, along with any related data files in a directory on your phone. I save all my data as layers in a single Geopackage file. I then use AirDroid to transfer files from desktop to phone. Note that in recent versions of Android (11+), security changes mean that there is an extra step required in QField to copy the files from the user-accessible folder to an app-accessible folder.
Then open the file in QField, and you can zoom in, out and around.
For a sample file with contours and streams, download the file below, and extract to a folder. The individual files can be copied to a directory on the phone and opened with QField
- pantoneys.zip (3.5MB)