On this page
A PDF page is not one rectangle. It can declare up to five, each with a different job: the physical sheet, the visible area, the bleed, the finished trim and the extent of the artwork. Readers pick one to display; printers pick others.
Once you know the hierarchy, a lot of strange behaviour stops being strange.
The five boxes
The MediaBox is required and defines the physical page. The CropBox is the region a viewer should display and print, and defaults to the MediaBox. The BleedBox is the area that extends beyond the trim so artwork can run to the edge; it defaults to the CropBox.
The TrimBox is the finished page after cutting, and the ArtBox is the extent of the meaningful content. Both default to the CropBox. A print workflow reads all five; a screen viewer usually reads only the CropBox.
- MediaBox: the sheet, required
- CropBox: what the viewer shows
- BleedBox: extra area for edge-to-edge print
- TrimBox: the finished, cut page
- ArtBox: the content extent
Who reads which box
A browser or phone viewer displays the CropBox. A desktop printer driver usually prints the CropBox. A professional RIP and imposition tool reads the TrimBox and BleedBox to place pages on a larger sheet with crop marks.
That is why a PDF can look cropped on screen and print with the full sheet: the viewer honoured the CropBox, the printer used the MediaBox, or the other way round.
Why your crop did not stick
Many “crop” features in viewers change only the display. They set a temporary view rectangle and forget it when the file closes. A real crop writes a new CropBox into the page dictionary, which is a small structural change and does not delete content.
That leads to the other surprise: cropping does not make a file smaller or remove the hidden content. Text outside the CropBox is still in the content stream, still selectable in some tools and still extractable. For redaction, that is a security problem, not an inconvenience.
Print, screen and the default
For office documents, all five boxes are usually identical and none of this matters. It starts to matter when a designer exports a print-ready PDF, when a printer asks for 3 mm bleed, or when a scan includes the black edge of the scanner bed.
A print-ready file needs a TrimBox that matches the finished size and a BleedBox 3 mm larger in Europe, or 0.125 inch in the US. PDF/X requires both to be present and consistent with the MediaBox.
Setting the boxes in practice
For a business document, leave every box equal to the MediaBox and forget they exist. For print, set the MediaBox to the sheet, the TrimBox to the finished size, and the BleedBox 3 mm larger on every side. The CropBox should match the TrimBox so that a screen preview shows the finished page.
Some tools expose only a crop control, which writes the CropBox and leaves the trim and bleed alone. That is fine for a screen document and wrong for a print job, because the printer still sees the original sheet and places crop marks around it.
Do it on this device
The tools this guide uses
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Questions this raises
4 answered- Does cropping a PDF delete the hidden content?
- No. It changes the CropBox, which is a display region. The content outside it remains in the file and can still be extracted, which is why cropping is not redaction.
- Why does my PDF print larger than it looks?
- The viewer is honouring the CropBox while the printer uses the MediaBox, or the print dialog is scaling to fit. Check both the page size setting and the boxes.
- What is bleed in a PDF?
- Extra artwork beyond the trim line, usually 3 mm, so that a slight misalignment during cutting does not leave a white edge. It is declared by the BleedBox and must extend past the TrimBox.
- Can I change the page size of a PDF?
- Yes, by scaling the page content into a new MediaBox. It re-renders or scales the content, so text can become smaller but remains vector if the tool scales rather than rasterises.
Written by the Ream team · last reviewed 29 October 2026
This guide was written and reviewed by the people who build Ream, not generated and published. It describes the interface as it behaves on current versions. If something has moved, tell us at hello@ream.tools and it gets corrected with a new review date.