There are quite a few services that should be able to solve this problem (turning a PDF into a web form and collecting signatures.) Here's a few of the services I'm aware of:
(I know about all these because I'm working on a PDF generation service for developers called DocSpring [1]. I'm also working on e-signature support [2], but that's still under development, and still won't be a perfect fit for your use-case.)
* https://www.hellosign.com/products/helloworks
* https://www.useanvil.com
* https://www.pandadoc.com
* https://www.pdffiller.com
* https://www.platoforms.com
* JotForm (https://www.jotform.com/help/433-How-to-Add-an-E-Signature-t...)
* https://www.webmerge.me
(I know about all these because I'm working on a PDF generation service for developers called DocSpring [1]. I'm also working on e-signature support [2], but that's still under development, and still won't be a perfect fit for your use-case.)
[1] https://docspring.com
[2] https://docspring.com/docs/data_requests.html