Two ways to set the edge
Snap to content edges scans the rendered page for its outermost non-white pixel and puts the box just outside it. That’s the one-click answer for a scan whose border you want gone entirely.
Uniform margin trims the same amount off all four sides: 5, 10, 15, 20 or 25 mm. That’s the right tool when the border is real, such as a physical margin you want kept, or when a page has a stray mark at the edge that would otherwise drag the snap box out to it.
The thing that catches people out
Snap finds the outermost mark on the page, and it doesn’t know what a mark means. A speck of dust, a punch hole, a scanned staple, a pencil note in the margin: any of them becomes the boundary, and the box ends up larger than the content you wanted to isolate.
The same applies to page furniture. A printed header or footer counts as content, so a snap that respects a running head stops well short of the text block. Mixed pages are the worst case. One page with a scan artifact and the rest clean will produce inconsistent crops unless you set each page individually.
Crop page by page, or all at once
Apply to every page is on by default, which is what you want on a uniform scan. Untick it and each page keeps its own box. That’s worth doing on anything scanned in a batch, or where one page has a manuscript note in the margin that its neighbors don’t.
- Apply to every page
- On by default. One box, written to every page in the document.
- Unticked
- Each page carries its own box. Switch between pages with the arrows, and the box you drew is remembered per page.
- Reset to full page
- Clears the box on the current page, and on all pages if apply-to-all is on.
- Page cards
- Show the current page count, the box size, and whether the crop is active.
Cropping changes the boundary, not the page
What the tool writes is a page boundary, the CropBox entry in the page’s dictionary. The page is instructed to display from one corner to another. The content outside it isn’t touched and isn’t deleted.
This is worth being precise about, because the tool is often mistaken for something that removes content. It doesn’t. Pull the boundary back, or open the file in an editor, and whatever was outside it displays exactly as before. If the goal is to make something unreadable, the redact tool is the one that removes text, and it works by redrawing the page as an image.
What it is useful for, and what it isn’t
- Scanned book or journal page
- Good fit. Removes the platen border and makes the text fill the screen.
- Slides exported to PDF
- Good fit. Wide presentation margins often leave a lot of dead space.
- A large-format drawing printed to A4
- Good fit for reading. It changes the page size your printer sees, which may be what you want and may not.
- Hiding a name or an amount
- Wrong tool entirely. Use redact, because a crop leaves the text in the file and selectable.
- Making a file smaller
- Marginal. The content is still in the file, so the bytes are still there. Compress is the tool for size.
- Straightening a skewed scan
- Not possible here. Crop changes the boundary; it can’t rotate the content inside it.