Do not impose the pages or save them as reader spread, this is not print ready.
We require a PDF consisting of single pages running from the front cover through to the back cover. If blank pages are needed in the final book they need to be included in the document.
For saddle-stitched books please remember that the number of pages in a book must be divisible by four (24pg/32pg/40pg etc). If your PDF has 10 pages you will need to add 2 blank pages to make it work.