You have probably heard many similar terms referring to PDF boxes. If you do not know what they mean, it is easy to mix them up and get confused. This article will help you understand what PDF boxes really are, how they are used, and why they are so important for viewing and printing files.
What exactly are PDF boxes?
The most common type of documents people send for printing are PDF documents. A PDF is the preferred format because it combines all components of your file, including fonts, color information, and other important settings. To have a print-ready PDF, you have to format your file to meet the printer's requirements. A PDF file describes the content and appearance of one or more pages. The pages are rectangular, so the file contains a definition of the physical size, height, and width of those pages. But the page size definition is not as simple as you might expect. The size of a PDF page can have different meanings (the printed page, the page visible in a PDF viewer, the cropped printed page, etc.), so the PDF standard defines a set of boxes. They are commonly called page boxes or boundary boxes. In short, you can see them as your PDF's internal measurements, which are used to define the page size.
PDF page boxes explained
Today we will describe all 5 of these page boxes: MediaBox, CropBox, BleedBox, TrimBox and ArtBox. PDF page boxes are used frequently in the printing industry. Let us start with the larger page size called the MediaBox. By default, the rest of the page boxes have predefined relationships in terms of their dimensions.
What is a MediaBox?
The MediaBox specifies the dimensions of the page. This, however, has different meanings for average users and printing industry professionals. For the average user, the MediaBox is equal to the actual page size. For professionals, the MediaBox is slightly larger than the actual page size. There are a few reasons for this, mostly related to industry processes and technology: it is useful to have a timestamp on the extended page that should not be visible in the final product; because the page is trimmed to the actual size, it is necessary to have some extra space beyond the actual page for any inaccuracies in the process, and so on. As you can see in Figure 1, all the other bounding boxes are inside the MediaBox.
Figure 1. The different page boxes that define a page's boundaries. The crop, bleed, trim, and art boxes will not normally extend beyond the boundaries of the MediaBox. If they do, they are effectively reduced to their intersection with the MediaBox.
What is a CropBox?
The CropBox is important for PDF viewer applications because it defines the page area to be displayed on the screen. The same dimensions are used when printing those documents from the PDF viewer application. The CropBox is not very relevant in a professional printing environment.
What is a TrimBox?
The TrimBox is a very important PDF box, as it defines the final shape and final cut of the printed material. If not set, the TrimBox will equal the CropBox.
What is a BleedBox?
In a professional printing environment, it is often necessary to have graphical elements that extend to the edge of the page, for example on front pages of magazines and posters. To compensate for imperfect page cutting processes, an extended graphics area is defined that is slightly larger than the TrimBox. This means that the boundaries of the graphics are about 3 mm outside the final page cut (TrimBox) to avoid blank spaces at the page edge. If not set, the BleedBox will equal the CropBox.
What is the ArtBox?
The ArtBox originally indicated the content area of the page (advertisements, images). Today it is most often used to describe the safety area, the parts of the page where it is safe to put images or other content. For example, the ArtBox can be used to restrict the text area in book design: if the text is too close to the book spine, it can be difficult to read.
A few common rules concerning page boxes
- Different boxes can be specified for each page of a PDF file
- The page boxes are always rectangular. However, the content described by a PDF does not need to be rectangular. For example, a label of a soda bottle described by the document can be oval in shape. The document page boxes, however, would be rectangular and would extend beyond the boundaries of the shape.
- Every PDF file must have a MediaBox definition. In general, the other boxes do not need to be defined for the document.
- The MediaBox is normally the largest page box in a PDF. The other page boxes can be up to the size of the MediaBox, but they are usually not larger. If they happen to be larger, a PDF viewer will still use the MediaBox values.
There are specific rules for PDF/X file formats:
- PDF/X-1a and PDF/X-3 compliant files must include TrimBox and BleedBox definitions in addition to the MediaBox.
- PDF/X-4 files require, in addition to the MediaBox, either a TrimBox or an ArtBox, but not both. If the BleedBox is defined, it must be smaller than or equal to the ArtBox or TrimBox, whichever is defined. If a CropBox is defined, the ArtBox or TrimBox, as well as the BleedBox, must extend beyond its boundaries.
- The PDF/X-4 standard forbids other boxes from exceeding the size of the MediaBox.
Now that you have a clearer picture of what PDF page boxes are and what they are used for, it will be easier for you to understand and select the correct settings for printing your document.
