Author of open-pdf-sign here. AdES alone is difficult to do without proper signatures and user verification.
Besides that, if Google would go for advanced electronic signatures, I'd expect it showing up in the EU Trusted List, which it isn't. So unless Google is not utilizing their own Google Trust Services certificate authority, I'd say it's unlikely that they will launch with AdES that are compatible with eIDAS.
Hi, Thomas (one of the creators) here.
This is actually the reason, why we are still supporting JRE8 with open-pdf-sign instead of having a JRE11 (or later) baseline.
We are offering a npm module as well (https://github.com/open-pdf-sign/open-pdf-sign-node). While that does not get rid of the JRE requirement, it makes integration in "modern" backends easier.