I recently had to implement a custom PDF viewer using PDF.js. It took a loooong time, because PDF.js is so confusing to work with. We went way over what we thought was a conservative time estimate. :(

It's not just a matter of documentation, but API design. Beyond the bare minimum functionality, you'll need bits and pieces of semi-public API.

For example, if you want to use the "text layer" (for selectable text) you'll need a CSS file which is inside the `web/` directory. But what's in `web/` is incomplete and not officially supported.