In case you do not have Android Studio and you do not want to install it, you may want to use gradlew scripts to build the application.
./gradlew assembleDebug should create ./app/build/outputs/apk/debug/app-debug.apk
From the root directory run the following command:
make droid
This will copy a secrets.xml file into the strings directory.
If you have a PSPDF license key then paste that value into the
pspdfkit_license_key entry in that file.