White Sides of Image Missing in Apple Motion

I am creating a book flipping animation in Apple Motion for a book that I have made, yet when I import my PDF, the white background to each page is missing, and has been replaced with a black (transparent?) background. Motion does still recognise the correct dimensions of the PDF.

For example, here is a screenshot of a page of the PDF, with a white background/outline, and then the border, image, etc.

And here it is in Apple Motion, with that white outline/border disappearing to black (transparent?), yet Motion still recognising its true edge:

Normally I can work these things out by myself, hence being a first time poster, but I've gone through the transparencies, masks, etc., and still cannot work out what to do. I'm sure it's an easy fix! Please help. :)