Does PDF.js raise an event as I browse between pages?

Yeah, there is an event named pagechange.

You can use it like that:

document.addEventListener('pagechange', function(e) {
  if (e.pageNumber !== e.previousPageNumber) {
    console.log('page changed from ' + e.previousPageNumber + ' to ' + e.pageNumber);
  }
});

Open the viewer and drop the code above into the console, and see what happens!

Now in v2.1.233 of pdf.js, the event is pagechanging.