The overall PDF reader/viewer functionality is encapsulated in a single ReaderViewController class. First, you initialize a ReaderDocument object with the file path to the PDF document (or unarchive an existing ReaderDocument object) and then initialize a ReaderViewController with this ReaderDocument object. The ReaderViewController class uses the ReaderDocument object to store information about the document and to keep track of document properties (i.e. current page).
✔ Sources: https://github.com/vfr/Reader