You could try and build your own electronic signature service, but that could be a lot of working depending on how fancy you need it to be. For example, if customers need to fill in dates, etc. For that, there are third-party services you can integrate with that offer the service, such as:
https://www.docusign.com/developer-centerhttps://www.hellosign.com/apiThere are many more. They include the ability for the user to make a signature with their mouse and overlay it somewhere on the document, then they sign it with the SSL cert. So you'd have some work to integrate your app with their API.
If its just a simple electronic signature, you could do it yourself. The iText java library (which I use in the ServoyGuy PDF Pro Plugin), includes that ability, but the feature isn't in the plugin. Its essentially a 2 part process:
1. You make any modifications to the PDF as necessary so that it "visually" appears signed, like adding a JPG of a signature over the signature line, or checking a box, something like that.
2. You use an SSL cert from a keystore you create, and use the iText java library to sign the document. Thats the final step that "locks" the document, and when you open it, it will show as signed. It also verifies it hasn't been modified. So of someone tries to tamper with it, they will get an error/warning message that its not a valid document any more.