I have just tested in Adobe's PDF viewer, and Preview on a Mac and it's fine.

Your PDF viewer doesn't support annotations, which is unfortunate because annotations are very much a core PDF feature.

I can see it not working in Google Chrome, but Google seem to have just fixed the bug for future versions (it'll be a few months at least I'd guess):
https://bugs.chromium.org/p/chromium/issues/detail?id=62625

I'll add a note in the tutorial.

Thanks. I have added this extension to Chrome in Chromebooks, and it appears to work if you download the PDF file, and then load it from local.

https://github.com/mozilla/pdf.js/wiki/PDF-Viewer-%28Chrome-extension%29

It should solve the problem for other Chromebook users for now.