Hi there!
PDF.js Express has a completely custom, open source UI, which is why it looks so different from PDF.js. We only use PDF.js as our core rendering engine.
It is very easy to customize our UI. For your use case of hiding certain elements, check out this guide.
This guide has instructions on integrating PDF.js Express in HTML.
PDF.js Express does support signatures out of the box, so I believe Express should be able to handle all your needs!
If you have any questions or run into issues, please let me know.
Thanks,
Logan