PDF.js dev here. I'm a bit confused on which part of internal API you would like to use? The way I think of it, there are really three API's in pdf.js: 1) Main thread API (api.js) which we base the version off 2) The code that runs in the worker 3) The viewer components (web/*)

Quite awhile ago when we decided what parts of the API to version, we thought more people would want to use #1. Now that the project is mature we could probably expose some more base the version off of that.

As for the "so that it doesn't look like PDF.js", we don't limit the API because of this. That suggestion (which I don't totally agree with) came from what we saw people doing, where they'd copy the entire viewer, when it'd probably be better to just let the user's browser choose how to show the PDF.