After you purchase a PDF copy of your book and download it, your PDF may display one page at a time when you first open it.
You can change the view so that two pages are displayed side-by-side, just like a book. These instructions explain how to change the view in Adobe Reader, a common program used to view PDFs that can be downloaded for free from Adobe.
In Adobe Reader, select View > Page Display > Two Page View.
Also select Show Cover Page in Two Page View.