After creating the cover in Illustrator, open in Photoshop with exact dimensions and then create your PDF there. Use High Quality printing.
If file won’t upload – you need to “flatten ” it.”
Make a copy of your PSD. Put it somewhere safe. Then flatten all the layers in your PSD before saving to PDF. Because Photoshop PDF files are editable and retain all the layers, the file size is usually about the same as the PSD file. Flattening the PSD removes all the editable data, reducing the file size dramatically.