What cropping a PDF actually does
Every PDF page carries a MediaBox — the size of the sheet it was laid out on — and may also carry a CropBox, a smaller rectangle telling viewers which part of that sheet to display. Cropping here writes the CropBox and changes nothing else.
That makes it the cheapest operation available. The content stream is untouched, so text stays selectable, fonts stay embedded, images keep their original encoding, and there is no quality loss of any kind. A 300-page document crops as fast as a one-page one, and the file size barely moves.
Cropping is not redaction
This is worth stating bluntly because it catches people out with real consequences. The content outside the CropBox is still in the file. It is not displayed, but it has not been deleted — anyone who opens the document in an editor and widens the box can read every word of it.
The same trap applies to drawing a black rectangle over a paragraph: the rectangle is a new object painted on top, and the text underneath remains fully selectable and fully searchable. Court filings and corporate documents have leaked exactly this way, repeatedly.
If something must genuinely be removed, remove it in the source document and re-export, or delete the whole page with the Remove Pages tool. There is no safe shortcut.
Rotation, and why it matters here
Scanners frequently save pages with a rotation flag rather than rotating the content itself. Such a page displays sideways but still has an upright coordinate system underneath, which means the edge you think of as “top” is not the top as far as the file is concerned.
This tool reads each page's rotation and re-maps your margins to match, so the preview and the result agree. Where a single document mixes rotated and unrotated pages, crop them in separate passes with the page selection field — the correct margins for one group are rarely the correct margins for the other.
Cropping a page that was already cropped
Margins are measured against what is currently visible rather than against the full sheet. A page that already has a CropBox has been cropped before, and applying another 10% takes 10% off what you can see now — which is what anyone adjusting a crop actually expects.
Common uses
- Removing the wide white margins from a scanned book so it fills an e-reader
- Trimming printer crop marks and bleed off a proof
- Cutting a running header or footer out of the visible area
- Making a document fit a tablet screen without shrinking the text
- Squaring up a photographed page before printing