This is the structure of QPDFSamplesViewer a sample project using qoppapdf.zip, Android Qoppa’s PDF toolkit, within Android Studio.

Download qpdftoolkit.zip (containing qoppapdf.jar, libs and assets folder)
Downnload qPDF_Samples_Viewer.zip

Make sure that the libs folder, jniLibs folder and the assets folder are in the correct location so that they are available as resources.

Structure in Android Studio
Note the location of the libs, jniLibs and assets folders.