flattening layers for outside printing

I'm ready to send in a children's picture book to CreateSpace for printing and I have a question about my file format. Createspace says layers should be flattened in the software that created them. I have about thirty pages of color illustrations I created in Painter, Paintshop Pro and CorelDrawX7. I'm also using CorelDraw for the actual book. Because I used three different programs to create the illustrations, it's going to be difficult (time consuming) to flatten the layers that way. If I convert all my illustrations which reside in my CorelDraw book to 300 dpi bitmap, is that the same as flattening the layers? I'm using the "Publish to PDF" option in CD. I notice that after converting my illustration to bitmap my pdf file is much larger. Thank you for your help.