Crop a PDF
Turn pages the right way up. Rotate the whole document, or pick out the pages that came in sideways and fix only those. The pages are copied rather than redrawn, so nothing loses quality. Nothing is uploaded.
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The pages
Load a PDF to see its pages.
Cropping hides, it does not delete
The sheet of paper is left exactly as it was and only the area a reader displays changes, so the trimmed content is still in the file. That makes a mistake easy to undo, and makes cropping the wrong tool for hiding anything.
How to use this tool
Drop a PDF on the box at the top. Set how much to take off each edge and every page shows the area you would keep, with the part being trimmed dimmed out.
The measurements are in points, which is what a PDF works in. Seventy two make an inch, so ten points is about three and a half millimetres.
Cropping hides, it does not delete
Worth being plain about, because it matters if you are trimming something you would rather nobody saw.
A PDF page has two rectangles. The media box is the sheet of paper. The crop box is the part a reader actually shows. Cropping sets the crop box and leaves the sheet exactly as it was, so the content outside is still in the file, just not displayed.
That is helpful when you change your mind, because nothing was destroyed and any software that lets you reset the crop box will bring it back. It is unhelpful if you were relying on the crop to remove something sensitive, because so will anybody else.
The top the reader sees
A page carries its own rotation, and this is where cropping quietly goes wrong in a mixed drawing set.
Take a page stored as portrait but displayed at ninety degrees. What the reader calls the top is, in the page own coordinates, one of the sides. Trim the coordinate called top and that page gets shaved down its edge while every other page in the set is trimmed across the head. One page looks wrong and the rest look fine, which is the hardest kind of fault to spot.
So the trims here are applied to the edge you actually see. Taking twenty points off the top reduces the displayed height by twenty points, whichever way the page is turned.
A worked check you can repeat
Take an A4 page, five hundred and ninety five points by eight hundred and forty two. Trim fifty off the top and the visible area becomes five hundred and ninety five by seven hundred and ninety two.
Now rotate that page a quarter turn and trim fifty off the top again. The crop box moves along a different coordinate entirely, and the displayed height still drops by exactly fifty. That is the same instruction producing the same visible result on a page that stores itself differently.
Trim sixty from the left and sixty from the right, and the centre of the page stays exactly where it was, which is what you want when the content is centred.
The geometry was checked against an independent implementation before publishing: thirty seven hand written cases covering each edge on its own, all four page rotations, pages that already carry a crop, symmetric trims, negative numbers that should do nothing rather than enlarge the page, and trims that would leave nothing. Then fifty thousand generated crops across six paper sizes confirming that the result never leaves the sheet, never grows back, always has real area, and that cropping twice by half is the same as cropping once by the whole.
Mistakes that catch people out
Using it to hide something. The content is still in the file. Cropping is for tidying margins, not for redaction.
Thinking in millimetres. The numbers are points. Ten is about three and a half millimetres, not ten.
Cropping a mixed set to a single measurement. Pages of different sizes lose different proportions. The preview shows each one.
Expecting the file to get smaller. It will not, or barely. Nothing is removed, only hidden.
Assuming a password protected file will open. It will not. Remove the protection first, in software that has the password.
Common questions
Is my PDF uploaded anywhere?
No. The file is opened by your browser, the cropped document is built here on the page, and the download comes from your machine. Nothing crosses the network.
Is the trimmed content deleted?
No. Cropping sets the area a reader displays and leaves the sheet itself alone, so the content outside is hidden rather than destroyed. Software that ignores the crop box can still reach it.
Can I undo a crop?
Yes, in software that lets you reset the crop box, because nothing was thrown away. That is a good thing for a mistake and a bad thing if you were trying to hide something.
What are the numbers measured in?
Points, the unit a PDF works in. Seventy two make an inch, so ten points is about three and a half millimetres.
What happens on a page that is already cropped?
It is trimmed further in from what you can currently see, rather than being reset to the full sheet. Margins somebody already removed stay removed.
Why was one page left alone?
Because the trims you asked for would have left nothing of it. Pages too small for the amounts are reported and skipped instead of being turned into something unopenable.
Related PDF tools and reference
Reference: ISO 32000, the standard that defines the media box and crop box used here.