Use the dApp here: https://jamesbachini.github.io/MetaSignature/
Creating a dApp to sign a legal document seemed easier than finding witnesses.
This simple script connects to metamask or another eth wallet and allows you to sign data such as the hash of a PDF legal file.
Source code in docs/index.html
https://github.com/jamesbachini/MetaSignature
Latest release: 1.0.0
Ethers.js signer.signMessage