Nice! I will give it a try later today.

For people who want a non web-based alternative, these days I use Xournal++ (https://xournalpp.github.io/) to do that type of edition locally.

What I am still looking for is a good way to clean scanned PDFs: split double pages, clean up text and make sure it is in lack and white, clean deformations and margin, cut and maybe rotate pages, compress the end result.