Get the coordinate of click inside iframe (pdf)

I display pdf documents in an iframe on my website (all documents are stored in the same domain).

I would like users to be able to click in the document to add comments at a specific location.
To do this, I am looking for a way to get the coordinate inside of an iframe (when we click).

It does not seem possible… However, there are paid solutions that offer this kind of functionality. Example : https://pdfjs.express/demo
So, what process is used?

Thanks for you help.