PDF crop / trim

The two big use cases:

• Strip scanner white space. Sheet-fed scanners produce PDFs with fat white margins. Cropping by 10–20 mm on each edge gives readers an immediately tighter, more readable document.
• Fit content to a target paper size. Press-ready books require an exact trim size — 6×9 in is the standard US paperback. Preset fit lets you centre your page content inside that box and discard the bleeds.
• Crop weird page sizes. Mixed A4 + US Letter bundles look ugly when printed; fit everything to A4 (or vice versa) for a uniform stack.

A PDF page carries several rectangles — media box (physical sheet), crop box (visible region), bleed, trim and art. This tool only changes the crop box. That means:

• Modern viewers (Acrobat, Preview, Chrome, Firefox) render only the cropped region.
• The trimmed content is still inside the file — it can be recovered by removing the crop box. If you need to delete it permanently, redact instead.
• File size is essentially unchanged: a crop box is two numbers per page.
• Text selection, search and embedded fonts continue to work exactly as before.

Trim margins is the right choice when every page has the same kind of border to remove. You enter four numbers (top, right, bottom, left) in your preferred unit — points, millimetres or inches — and they're applied to every selected page. Margins are subtractive: 0 on all four sides leaves the page unchanged.

Preset fit is the right choice when you want a specific final size. Pick the preset (Letter, A4, Legal, A5, 6×9 book) and an alignment — centred is the default; corner alignments are useful when content sits flush to one edge of the original.

If the preset is larger than a particular page, that page is reported in the error and skipped.

The PDF is parsed in a Web Worker; pdf-lib sets a new CropBox on each target page; the modified PDF is offered as a download. There's no network round-trip and no server-side copy at any step.

• Password-protected, digitally signed, or archival PDFs may need a specialist workflow before editing.
• Large scans can use a lot of memory, especially on phones or older laptops.
• Check the downloaded file before replacing the original, because compression, OCR, or conversion can change visual details.