Why crop a PDF?
Not every PDF is sized the way you need it. Scanned documents often carry wide white borders left over from the scanner bed, exported reports can leave awkward whitespace around the actual content, and pages pulled together from different sources into one merged PDF rarely line up. Cropping trims those excess margins off every page so the content fills the page better - useful before printing, before sharing, or just to make a document easier to read on screen.
How much should I trim?
Start with a lighter preset if you're not sure - it's easy to re-upload and try again with a bigger trim, but a crop that's too aggressive can cut into your content. The custom option gives you separate control over each side, which is handy when a document has uneven margins, like extra space at the top for a header that's no longer needed.