What this tool does
Cropping a PDF means setting a box, not deleting pixels, and the two are easy to confuse. A cropped PDF still contains everything it always did, and a reader showing the cropped area hides the rest rather than removing it. That is usually exactly what you want for printing, and it is a genuine warning if the reason you are cropping is to hide something.
How it works
A PDF page has several boxes and they are not interchangeable. The MediaBox is the physical sheet. The CropBox is the area a reader displays as the page, so the difference between the two shows as a grey margin around the content. The TrimBox is the finished size after guillotine trimming, and it is the one a print shop actually reads. The BleedBox extends past the TrimBox to allow for ink spread. Setting only the CropBox gets a PDF to look right on screen and display correctly everywhere, which is what most people need.
Coordinates run from the bottom-left corner with y increasing upward, which is the opposite of every image tool. It is the single most common trap in PDF work: a crop defined the way an image tool would be, pasted straight into a PDF, silently selects the wrong region of the page. The tool takes measurements the way people state them, from the top-left, and converts them, so the number you see is the number you meant.
Auto-trim finds the edge for you. It looks for the bounding box of everything that is not background colour, which handles the white margin a flatbed scan leaves around a page and the dead space around a screenshot. The tolerance is adjustable, because a scanned page is rarely pure white: an off-white or slightly grey border needs a wider tolerance to be included, and setting it too wide pulls in paper texture and crops too little. The detected edge is shown as a box before it is applied.
Because a crop is a box and not an edit, it is completely reversible and it loses nothing. The original content is still in the file, still present, and still recoverable by removing the box. That is also why this is not a way to remove a name from a document: the text is still there and still selectable, and a rectangle drawn over it would be no better.
Worked example
A flatbed scan has left a wide white border on every page, and the document is going to a print shop that needs the trim size.
- Use auto-trim, which detects the edge of the scanned paper and proposes a box
- Adjust the tolerance if the detected edge is inside the paper rather than on it
- Check the reported margins in millimetres, which is what the print shop will ask for
- Apply, and set the trim box as well if the file is going to press
A document with tight pages and a correct trim size, produced by setting boxes rather than by deleting content, so nothing has been lost and the crop can be undone.
Accuracy and limitations
- Cropping sets a box. It does not delete anything, so text outside the crop is still in the file and still selectable. It is not a redaction method.
- The grey area a reader shows around a cropped page is the part outside the crop box. That is normal and does not print.
- Auto-trim works on a downscaled copy of the page, so the detected edge is accurate to roughly a point rather than exactly.
- A page whose content genuinely reaches the paper edge has nothing to trim, and the box will be the whole page.
Frequently asked questions
- How do I crop a PDF page?
- Enter the margins to remove, or drag a box on the preview, and the tool converts them into the bottom-left coordinates a PDF crop box requires. The reported margins are given in millimetres, which is the unit a print shop will ask you for.
- Does cropping a PDF delete the content outside the box?
- No. A crop sets a box that a reader treats as the page, and everything outside it stays in the file, still present and still selectable. This is what makes cropping reversible and lossless, and it is also why cropping is not a way to remove information from a document.
- What is the difference between a crop box and a trim box?
- The crop box is what a reader displays as the page, and it is the one that controls what you see on screen. The trim box is the finished size after a guillotine cuts the paper, and it is the one a print shop reads. Setting the crop box is enough for most uses; setting the trim box too is what you want when the file is going to press.
- Why are PDF crop coordinates measured from the bottom?
- Because the format defines the origin at the bottom-left corner with y increasing upward, which is the opposite of every image format. It is the most common cause of a crop landing in the wrong place, so this tool takes measurements the way people actually state them, from the top-left, and does the conversion itself.
- How do I remove the white border around a scanned page?
- Use auto-trim, which finds the edge of the content and proposes a box. If the detected edge sits inside the paper rather than on it, widen the tolerance, because a scanned page is rarely pure white and a narrow tolerance will not treat a slightly grey background as background.
- Can I undo a crop?
- Yes, trivially, and that is the point of doing it as a box rather than an edit. Removing the crop box restores the full page, because the content was never removed. Start again from your original file if you need a different crop.