Crop a PDF

Page boxes: MediaBox, CropBox and what a reader actually shows

A PDF page does not have one size. It has several rectangles, called boxes, and they do different jobs.

• MediaBox is the full sheet the page was made on. Every page must have one. An A4 page is 595 by 842 points, since a point is 1/72 of an inch (about 0.353 mm).
• CropBox is the area a reader displays and a printer prints. If it is absent, it defaults to the MediaBox.
• TrimBox, BleedBox and ArtBox are used in print production to mark where the finished page will be cut and how far ink extends beyond it.

Cropping in the ordinary sense sets the CropBox. Nothing on the page moves and nothing is resized: the reader simply shows a smaller window onto the same page. This is why cropping is quick and does not touch the quality of images or text.

It also has a catch. Because the content outside the window still exists in the file, a program that ignores the CropBox, or a user with an editor that lets them widen it, sees the lot again. Some converters and image-extraction programs use the MediaBox, not the CropBox, so a cropped page may come out uncropped from them. That is the reason for the delete option.

The delete option

With Delete content outside the frame ticked, the tool also removes page content that sits wholly outside the frame and brings the page’s outer boundary in to match. Because content that straddles the frame’s edge cannot be sliced without redrawing it, anything that crosses the line stays. That has a consequence for scans: a scan is a single image that covers the whole page, so it always crosses the frame, and cropping a scan will hide the margins but cannot shrink the file. For a scan, think of cropping as a viewing change only.

Working in millimetres: common sizes and what to trim

To work out a crop, subtract what you remove from the page size. An A4 page with 15 mm taken from every side leaves 180 by 267 mm. Remember that 1 mm is about 2.83 points, so a 10 mm trim is about 28 points.

The tool names the page size it sees if it is within about 2.5 mm of a standard size, otherwise it shows the dimensions. The word “portrait” or “landscape” follows what you see on screen, after any rotation.

When cropping helps, and what to use instead

Cropping is the right tool for

• Scans with dark borders or scanner-bed edges. Trim the shadow from the page edge so printing uses less ink and the page looks tidy.
• Binding shadows in book scans. Odd and even pages have the shadow on opposite sides, which is what the odd and even options are for.
• Slides printed with a wide margin. Take the border off so that the content fills the screen on a small device.
• Exported reports with white strips. Remove an extra centimetre round each page so a table fits better on a phone.
• Screenshots pasted into PDFs with unwanted edges of browser chrome or taskbars.

Better handled another way

• Removing pages altogether: Organize pages or Extract pages.
• Straightening a crooked scan: cropping does not rotate. Use Rotate PDF for 90 degree turns; fine skew needs a scan-cleaning program.
• Hiding private content at the edge of a page. Cropping hides, and unless you tick the delete option it does not remove. For confidential material use Redact PDF.
• Making the file smaller. Cropping mostly does not. Use Compress PDF.
• Resizing the page to a standard paper size. That scales the content, which cropping does not do; use your print dialog’s fit-to-page option.

Odd pages, even pages and ranges: how to crop a book properly

The crop applies the same four margins to every page you select. Selection is by page number, counting from 1. That makes an uneven book scan a two-pass job, because the shadow from the spine is on the left of odd pages and the right of even ones (or the reverse, depending on how the book was scanned).

1. Set the left margin that removes the shadow on a typical odd page. Choose Odd pages. Crop and download.
2. Open the result in Crop PDF. Set the right margin to remove the shadow on a typical even page. Choose Even pages. Crop and download.

If your scan has a cover that is a different size from the body, handle it separately with a page range such as 2-end so the cover is left alone. The same page range logic works for appendices. You can run several passes, each on different pages, since each pass writes a new file. Cropping an already cropped page moves the window further in from where it is now, so a second pass on the same pages takes more off.

When pages are different sizes, the same millimetres come off each, so the pages end up different sizes after cropping too. The frame on screen is only exact for the page you are looking at.

Doing it in Acrobat, Preview, Word or Google Docs instead

• Adobe Acrobat Pro has a Crop Pages tool with margin controls, a page range and, on top of the CropBox, a choice to remove white margins automatically. This tool does not detect or remove blank areas on its own; you set the margins.
• Apple Preview has a Crop command that works on a selected rectangle. Check how many pages it applies to in your version, and look at the saved file in a second reader before it goes to a printer.
• Word can crop pictures, not PDF pages. Opening the PDF in Word to change its margins rewrites the layout.
• Google Docs and Drive do not crop PDF pages.
• Printing to PDF with scaling can trim by scaling the page, but it changes the page size and sharpness of text in the process.

Where this tool is strongest is the batch case: the same trim on every page, odd pages or a range, typed in millimetres, with no upload.

Three worked examples with numbers

The figures are round numbers to show the arithmetic. They are not measurements.

A 40-page A4 book scan with spine shadow

The shadow is about 14 mm wide on the outer edge of the page it falls on. Pass one: left 14 mm, Odd pages. Pass two: right 14 mm, Even pages. Odd pages are now 196 × 297 mm and even pages are 196 × 297 mm. Pages are the same size, which prints neatly. Because the scan is one image per page, the file size does not drop.

A 16:9 slide deck with a 10 mm white border

A 10-slide deck exported to PDF at 338.7 × 190.5 mm has a 10 mm white border on every side. Set all four margins to 10, apply to all pages. Each slide becomes 318.7 × 170.5 mm, a slightly different shape (about 1.87 to 1), so check how it looks in presentation mode. If it must stay 16:9, remove proportionally: 10 mm from the sides needs about 5.6 mm from the top and bottom.

A report with a private note in the margin

A one-page report has an editor’s name in the right margin that you want gone. If you crop 25 mm from the right and leave the delete option off, the name is hidden but still in the file, and a program that ignores the CropBox shows it. Tick the delete option, and content wholly outside the frame is removed. If the name partly overlaps the frame, it survives; for sensitive material use Redact PDF and check the result.

Limits and edge cases

• Rotated pages. Margins are measured on the page as you see it, whatever its stored rotation, so “left” is the left of what you see.
• Minimum page. The tool will not crop so far that less than 10 mm of width or height is left.
• Existing CropBox. If the file already has a smaller CropBox than its sheet size, your margins are measured from the visible area, not from the hidden sheet.
• Links and annotations in cropped-away areas. A link or note whose position is wholly outside the frame is not visible. Whether it still exists depends on the delete option.
• Bookmarks and text search. They keep working, as the page content is unchanged unless you delete outside the frame.
• Print shops. If a file is going to a commercial printer, ask whether they use the CropBox or the TrimBox. Cropping here sets what appears on screen and on a desktop printer.
• Signed PDFs. The file is rewritten, so any signature on it will not validate on the copy.
• It does not resample. Image resolution is unchanged. A scan at 300 dpi cropped to A5 is still 300 dpi.

Privacy: scans of letters, IDs and books stay on your device

The documents people crop are often scans of paperwork: a utility bill, a passport photo page, a signed form. The preview is rendered in your browser, the frame you drag is just numbers in the tab’s memory, and the cropped file is written on your own device. No part of the page is sent anywhere.

The verification guide shows how to watch your browser’s network activity while you crop, or to repeat the whole job with the connection off.