It's a Dark Fantasy RPG system born in a PBF game which you will probably find confounding because it's entirely written in Polish. This repository contains its rule books source and additional stuff used along side.
First of all, books use CSS Paged Media with paged.js.
To compile it by yourself, you need install NPM. Then run following commands:
$ npm install
$ npm run corebook
Final PDF files can be found in books/build directory.
The books content itself are under the Copyright license, and other underlying source code used to format and display that content, etc. is licensed under the MIT license.