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. It also allows for viewing of GeoTIFF files, in case you want to load topo maps, which are often made available in this format.

QField comes in both Android and iOS versions.

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)

First save GeoTIFF (.tif) file(s) in a directory (eg geotiffs) on your phone.

Then go to Local Projects & Datasets and click the Add (+) button.

Select Import project from folder, and then navigate to the folder where you've saved the files. Select Use This Folder, and allow QField access. If the folder already exists, QField will ask to confirm to overwrite.

Once done, you can navigate to Imported projects → geotiffs, and open the dataset.