Some publications still work with hard print and their PDFs will contain crop marks that need to be removed so they are not displayed in your Flipbook. The Flipbook system requires your PDF to contain pages that are all the same size.
There are multiple ways to remove crop marks but here is a way to do this using Adobe Acrobat:
Navigate to a page that needs crop marks removed and go to Document > Crop Pages.
Using the Margin Controls, you can set the cropped page area. Make sure the cropped page will not show any of the crop marks. Also make note of the cropped page size as this is the page size you will need ALL pages to conform to.
If every page of your PDF has the same crop mark dimensions, you can apply the crop to a Page Range of "All".
If your PDF has different cropping dimensions you will need to look at each page of your publication and crop accordingly.
REMEMBER ALL PAGES MUST BE THE SAME DIMENSIONS!
If your PDF is in double page spread, please crop to single page.