how to remove drop shadow from rectangles - Preview

I'm makes some changes on a PDF using the Annotate feature in PREVIEW. I've used a white rectangle. However, it comes with a drop shadow, which looks terrible on the document. Is there any way to remove the shadow? I've looked on preferences and searched the internet, nothing. Attached is screenshot.

PS. This might be the wrong community, but I couldn't figure which one "Preview" questions belong to.