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.