Add Annotations in Xamarin forms Application

Question

In my xamarin forms application, I'm using WebView to display a PDF file from a URL.

I've followed the approach suggested in Display PDF File in mobile app from an url

How can I add annotations in the pdf file displayed in WebView & save it in Xamarin forms. It can be achieved using Sync Fusion & PDFTron, but is there a free approach to achieve this functionality.

Any help is appreciated!