Implementing embedded PDF signature with Drag & Drop funcionality

I have experience in web development, but I have one task, but I have no idea what tools to use to implement such funcionality.

I need similar features to rightsignature.com .

1. Edit PDF document, add text fields, signatures ....
2. Send document for submission to others
3. Get results of submission

As far as I explored their implementation, they are transforming a PDF file to an image and than just display it on a web page allowing to drop text areas, signature fields and others. All fields are simple div elements. And background is a pdf image.

So I am not sure if this is the best solution. But at least it has a great backward compatibility. I don't to need the same features as this service provides only few simple features.

1. What are possible solutions of solving this problem ?
2. What frameworks are available for such procjec (JS, drag and drop, PDF converters ....)
3. Other suggestions and ideas

I would be grateful for any help

Thanks