The issue looks more like that the annotation (Widget in this case) selection tool (the blue square with 5 blue circles) is not scaling.
In the "Normal unzoomed" image, the annotation goes from the "Google Drive" to "Dropbox" icons, but on your zoomed in ones, the rectangle is about half as wide.
The actual signature (the green ink) is the correct size in all images, it should not change size when you zoom in/out. You can see that in both images, the top is just below "Google Mail" and the bottom is over "SELECT".
I would check what the behaviour is for our iOS PDFViewCtrl sample, and if/how it supports zooming while you have an annotation selected or are creating an annotation.
If this is a new feature you are adding, then you will need to update your "tag" when a zoom even occurs, which there would be a way for you to register for.