How to add custom annotation with help of Rect points in pdf doc?

Hi,

I have created Custom Annotation using stamper class. My app requirement is the size of stamp annotation should be same when i zoom in or zoom out. The stamp size should not vary as per pdf document zooming. Is there any way to fix the size of custom annotation even in zoom in?

I have attached two images, how it looks now. Here, the annotation size is increasing when i do zoom in.