PDF crop boxes explained: media, crop, trim and bleed

To make a scanned page easier to read, you can trim its visible border with the PDF crop tool. It changes the page’s CropBox, the rectangle a viewer normally displays. It leaves the content outside that rectangle in the PDF.

Use this for presentation, such as reducing empty margins. If the border contains a name or account number that must be removed, use content redaction instead.

Crop one page before applying it to a document

1. Open the PDF and choose a representative page.
2. Drag over the area you want to keep, or enter the top, right, bottom and left margins. The default unit is millimetres.
3. Set Apply to to This page only for a first check, or choose a page selection such as 1-3,5.
4. Choose Crop & download, open the saved copy and check all four edges.
5. Apply the margins more widely only when they suit the other pages too.

The tool applies the same physical margins to each selected page. A 20 mm margin takes proportionally more space from a small receipt than from an A4 report. Check mixed sizes and landscape pages individually.

The implementation accounts for quarter-turn page rotation. Already-cropped files need extra care: the preview uses the current visible page, while the export calculates new margins from the MediaBox. A new crop may therefore differ from the area you expected from that preview. Prefer the uncropped source when available, and check a single-page export first.

What the five page boxes mean

These are boundary definitions, not five layers or a fixed set of nested rectangles. Several can have the same values or use defaults.

Box	Purpose
MediaBox	Required page boundary, which may include print marks and extra space
CropBox	Region normally displayed or printed; defaults to MediaBox
TrimBox	Intended finished page dimensions after trimming
BleedBox	Production area including content extending beyond the trim edge
ArtBox	Extent of meaningful content, including relevant whitespace

TrimBox, BleedBox and ArtBox default to CropBox when absent. They are most useful in print-production and page-placement workflows. The crop tool here changes CropBox; it is not a prepress editor for setting all five.

Why cropping usually does little for file size

Changing the CropBox does not downsample a scan or delete its hidden pixels. The image and text objects are still stored in the document. Saving can change the file’s size through rewriting, but the smaller visible rectangle itself is not a useful compression method.

The PDF compressor instead renders pages into new JPEG images. That can change size and quality, and it removes the existing text layer. Choose it only if a picture-only copy suits the task; it is not the next required step after cropping.

What stays, and what needs checking

The crop operation does not rasterise the page. Existing text and vector drawings remain as objects, and the MediaBox and page order are retained. Hidden text may still appear in search or text extraction, and another editor can reveal it by changing the page boundary.

The file is saved again, so avoid assuming every property is byte-for-byte unchanged or every viewer will display it identically. Check links, annotations and printing behaviour if you rely on them. Keep the original of a signed or production-ready document.

If you also need to remove or rearrange whole pages, do that in the PDF organiser and check the page positions before cropping the resulting file.

Removing sensitive content needs a different workflow

Use a PDF editor’s dedicated redaction feature, apply the redactions and inspect the saved result. Alternatively, remove the information in the source document and export again. A rectangle drawn over text can leave the original objects underneath, just as a CropBox can leave them outside the visible area.

A new file made solely from reviewed, opaque-covered page images omits the source’s hidden text objects, but loses search and other document features. It still needs a check for visible clues and metadata. Cropping or compressing alone is not a verification of redaction.

Sources and further reading

• ISO 32000 / PDF specification — page boundary definitions
• PDF Association — PDF format and production resources
• pdf-lib documentation — library used by this tool
• pdf-lib PDFPage reference — box meanings, defaults and setCropBox

Last reviewed 2 October 2026.