JHipster embraces Angular, but it has it's unique way to do so. Angular hides the Webpack files from the (unexperienced) developer. JHipster exposes the build process. That's great because you've got more options to fine-tune the process, but it's also bad news for library developers because their standard documentation doesn't work.
This repository is a collection of JHipster projects running ngx-extended-pdf-viewer. So you've got a running project. That's a great way to get started with ngx-extended-pdf-viewer (or any project, FWIW), or to find the differences to your project if you've run into trouble when trying to install my pet library.
The good news is that things have become pretty simple with JHipster 6.6. It's almost like the Angular CLI approach, apart from different folders and using the Webpack copy plugin. Two easy steps to get everything up and running, as described at https://pdfviewer.net/getting-started.
Enjoy!