I am creating a pdf for a book created in Designer Pro X9. I need to print it on Createspace (Amazon's print on demand system). I have tried exporting as a pdf with images at 300 dpi.
When objects are flattened by the PDF process, a thin line appears around objects with a transparent background.

The lines do not show up when I open the pdf with acrobat on my pc;
the lines do show if I read the pdf on my ipad;
the lines show on the pre-press pdf.

Is this a known problem and how can I avoid this? Thanks