In https://tex.stackexchange.com/a/85676/4011, it is illustrated of how to annotated a pdf by using the latex package
pdfpages, loading the pdf as a background layer and then use
tikz nodes to make annotations at coordinates
(x,y) (compare here for similar approaches). The problem is that you need to get the coordinates of where you want to make the annotations manually by reading it from a overlayed coordinate system.
How can I use pdf-tools to get the coordinate automatically by just clicking on the place where I want to insert the annotation?