Yes, the elements are painted in our internal logic. Unfortunately, I cannot provide you with a workaround in this case.
Let me know if you have any other questions.
Specifically, this is a single-pixel border around some elements, regardless of zoom level.
I thought I might be able to get around this by changing the LayoutRounding of the control but it looks like the PDFViewer creates it's own Canvas internally which I can't modify.