Thanks for your response.
We are having different challenge here right now. Currently we have implemented pdfjs library in our application with xfdf file storage on our server without using any kind of database which is working fine but not having live collaboration feature.
To have that live collaboration feature, we decided to move this functionality from xfdf to google firebase(Firestore) where all the annotations will be saved realtime and multiple users can work together on single document. We are able to achieve that functionality as well. but we are not sure that how can we move existing annoted files into google firebase because we are able to load xfdf annotations but they are getting saved to firebase until we move/add annotations to the file.
It means, we need to do some operations to get it saved to firebase. Due to this, we are not sure how we can save all annoted files into firebase using any api so that we can deploy use real time collaborated pdf features in production and deprecate old xfdf file saving approach.
Let me know if you still have any doubt.