MediaBox, CropBox, BleedBox: Which Page Box Am I Looking At?

A PDF page is not one rectangle. The format stores several page boxes so printers, viewers, and editors can disagree about "the size of the page." When a crop looks wrong, or a print shop asks for bleed, you are usually arguing about which box is in play.

The boxes, in plain terms

• MediaBox: the full page canvas; the largest box that defines the media size. Every page is expected to have one.
• CropBox: the region viewers use as the visible page. If omitted, it defaults to the MediaBox.
• BleedBox: the region that includes bleed past the trim edge for commercial print.
• TrimBox: the intended finished size after cutting.
• ArtBox: the extent of meaningful content (less common in office PDFs).

Viewers almost always paint the CropBox. Prepress software cares about BleedBox and TrimBox. Confusing them produces the classic "I cropped it but the printer still sees the old margins" report: the on-screen window moved, but MediaBox or TrimBox still advertise the old canvas.

Coordinates are in PDF user space. One point is 1/72 inch. Origin is the lower-left of the box being described, not the upper-left of a screen window. Tools that take "X, Y, width, height" are describing a rectangle in that space.

Cropping changes the visible boundary

On PDF123, Crop takes X, Y, width, and height in points and writes that rectangle to both MediaBox and CropBox on every page. Viewers that honor CropBox and tools that read MediaBox therefore see the same page size after the job. Content outside the new box is hidden from view; the underlying content stream is not necessarily deleted. Geometry first; shredding unused drawing operators is a separate job.

Other apps sometimes change only CropBox and leave MediaBox alone. Reopening that file elsewhere can look like a "failed" crop when it is really a box mismatch. Inspect which box each tool displays before assuming the file is broken.

Crop does not rewrite BleedBox, TrimBox, or ArtBox. If a print workflow still reads those dictionaries, they may still describe the pre-crop page until a prepress tool updates them. Office-style "trim this margin for email" jobs usually care only about MediaBox/CropBox alignment—which is what this Crop path sets.

Scale is a different lever

Scale pages changes page size (A4, Letter, a scale factor) rather than trimming a window inside the current canvas. It also sets MediaBox and CropBox to the target size, and wraps page content in a scale transform so the drawing fits the new canvas. Use crop when you want to keep a region and hide the rest. Use scale when the whole page should map to a target paper size.

Cropping a letter-size page to a 200×200 pt window and scaling that same letter page to A4 are not interchangeable. One discards (from view) everything outside a rectangle; the other remaps the entire page into a new media size.

Diagnosing a “wrong size” file

When a print shop says the page is still letter-size after you “cropped” it in another app, export or inspect MediaBox and CropBox separately. A CropBox-only change can look correct in a viewer while MediaBox still reports the old canvas to imposition software. PDF123 Crop avoids that split by writing the same rectangle to both.

If pages look clipped after a merge or import, check whether an upstream tool set CropBox smaller than MediaBox without updating both. Get Info reports page count and encryption, not box coordinates—use a PDF inspector or the crop UI’s point fields when you need the numbers.

Practical takeaway

Ask "which box am I looking at?" before trusting on-screen millimeters. If two tools disagree on page size, compare MediaBox vs CropBox before blaming corruption. For interactive trim: Crop a PDF, no account required. For automation, the same geometry fields ship on POST /api/v1/general/crop; see Developers.