The CropBox defines the region that the PDF viewer application is expected to display or print.

This means that PDF files used in graphic arts usually have a MediaBox which is larger then the trimmed page size. For prepress use, this is not the case as we prefer our pages to be defined slightly oversized so that we can see the bleed (Images or other elements touching an outer edge of a printed page need to extend beyond the edge of the paper to compensate for inaccuracies in trimming the page), the crop marks and useful information such as the file name or the date and time when the file was created. For the average user, this probably equals the actual page size.

The MediaBox is used to specify the width and height of the page.

These are called the boundary boxes or page boxes: There can in fact be up to 5 different definitions in a PDF that relate to the size of its pages. That page size definition is not as straightforward as you might think. It also contains a definition of the physical size of those pages. A PDF describes the content and appearance of one or more pages.

Adobe acrobat pro 9 pdf/x-1a settings