By default the TrimBox equals the CropBox. The imposition programs and workflows that I know all use the TrimBox as the basis for positioning pages on a press sheet. Contrary to the CropBox, the TrimBox is very important because it defines the actual page size.
The TrimBox defines the intended dimensions of the finished page. By default the BleedBox equals the CropBox. Most prepress systems allow you to define the amount of bleed yourself and ignore the BleedBox. It is nice to know the size of the BleedBox but it isn’t that important in graphic arts. Usually the BleedBox is 3 to 5 millimetres larger than the TrimBox. The BleedBox determines the region to which the page contents needs to be clipped when output in a production environment. The GWG industry association recommends not to use the CropBox at all. For prepress use, the CropBox is pretty irrelevant. Acrobat uses this size for screen display and printing. The CropBox defines the region to which the page contents are to be clipped. The other page boxes can equal the size of the MediaBox but they cannot be larger. The MediaBox is the largest page box in a PDF. Pitstop professional and imposition studio pdf#
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. There can be up to 5 different descriptions in a PDF that relate to its size. The exact size of that page is not as straightforward as you might think. 
The PDF page boxes: MediaBox, CropBox, BleedBox, TrimBox & ArtBoxĪ PDF describes the content and appearance of one or more pages.
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