Mygazines system requires a PDF that is in single page format.   A double page spread will be treated by the system as a single page and will not display your publication correctly.  Any double page spread pages must be cropped to 2 separate pages of the same size.  We always recommend Adobe Acrobat for this kind of formatting.

If any of your PDF pages are double page spread like this:

You will need to crop this page and split into two pages:

If your entire document is in a two page spread, you can apply the left hand crop for all pages and save a left hand page PDF.  Do the same for the right hand side pages then you will have to merge the two files, ensuring your pages get put into the correct order in one PDF.

You can see that the above pages also contain crop marks which are used by paper printers.  

See this article for help on how to remove crop marks