Honestly I don’t really know. I don’t think it has to be but you’ll need to research what kind of certs you’ll need for your purposes and what options exists for a CA.
My overall point was more that this isn’t really a “just setup Apache” web service, it’s a fair bit more involved since your looking for (I’m assuming) legally valid digital signatures which means they need to be secure, unique, identifiable, and adhere to non-repudiation. Services like DocuSign do all of that work already so unless you truly want open source because of passion or principles, this isn’t really a great area to try and roll your own without some experience and expertise.
Anything you use will need to generate secure certs for each user that signs which will need to be tied back to a root CA that is trusted by the clients device they are reading the signed documents on. Are you open to hosting all of the needed pieces to keep that working including buying the needed certs to chain trust?
Paper goods meaning napkins, paper plates (mostly for food prep), paper towels, etc. agreed 100% on actual paper for writing.