Which PDF box sets the page size: MediaBox, CropBox or TrimBox?

A PDF page can carry five boxes. The MediaBox is the whole sheet, the CropBox is the visible region that viewers show, the TrimBox is the finished size after cutting, the BleedBox is the area to clip to in production, and the ArtBox marks the meaningful content. For print, the TrimBox usually defines the finished size.

Key points

• Each box is a rectangle on the page. They are separate settings, so a file can carry several different sizes at once.
• Your viewer shows the CropBox. A printer's workflow can look at the TrimBox and BleedBox instead, which is how the same file ends up with two sizes.
• If a PDF has no TrimBox, nobody can read the finished size from it. A checker can only take the page size or ask you to type the finished size.
• A page that includes bleed is bigger than the finished size by design, so a larger MediaBox is not automatically a mistake.
• Rotation and mixed page sizes change what you see, so check them before blaming the boxes.

What the five boxes are

The definitions below follow Apple's Core Graphics documentation, which describes each box in the same terms as the PDF format.

Adobe describes the purpose of the bleed area in the same spirit: it holds objects that extend past the page edge, so there are no white gaps after trimming. If you want the numbers to use, see How much bleed do you need for printing?. Acrobat Pro includes a tool for defining the crop, trim, bleed, art and media boxes on a page, so the boxes in a file are settings that can be set or changed, not facts about the artwork.

Why the sizes disagree

• The CropBox is smaller than the page. The viewer shows only the CropBox, but the file contains a larger page.
• There is no TrimBox. The finished size has to be guessed from the page size.
• The page includes bleed. A 216 × 303 mm page is an A4 page with 3 mm bleed on every side. It looks wrong next to a 210 × 297 mm requirement until you know that.
• Pages have different sizes. A cover spread or a placed page from another file is a common cause.
• A page is rotated. The stored size and the displayed size are different by 90 degrees.
• UserUnit is not 1. UserUnit is an optional setting that scales a page. Our table shows it, and 1 means no scaling.

What we tested

We generated six small PDFs and ran them through the PDF Page Size Checker. Nothing in them is customer content.

Notice that the third file passes with no TrimBox at all. It passes only because we told the checker the finished size. Without that, it would have had no way to know that the extra 3 mm is bleed and not a wrong page size.

How to find out which size is right

1. Ask your printer for the finished size and whether they want bleed included in the PDF.
2. Open the page boxes of your file rather than reading the size from a viewer. Our table lists the MediaBox, CropBox, TrimBox, BleedBox, ArtBox, rotation and UserUnit for every page.
3. Enter your finished size and compare. If the page equals finished size plus bleed, that is expected.
4. If the CropBox is smaller than the page, or the sizes are mixed, re-export from the source file with one page size and the boxes you intend.

Limits

The checker reports the boxes that are stored in the file. It cannot tell you which box your printer's software will use, and it does not look at your artwork, so it cannot tell whether the design reaches into the bleed. It also does not fix the file. Our test files are generated documents with simple page content.

What to do next

Ask the printer which size they expect, then compare it with the boxes in your PDF. If the PDF also contains images, read What DPI do you need for printing?. For how these checks work and what they leave out, see how Check Before Submit works.