Two ways to crop — and one honest warning
Auto mode renders your pages, finds where the actual content ends, and trims the white margins around it — with a small safety pad so the crop never touches the ink. It is the fastest fix for reading papers and books on an e-reader, where centimetre margins waste half the screen. Manual mode takes exact millimetres for each side instead, for when you know precisely what should go.
The honest part, and it matters: cropping changes which area of the page is displayed. The content outside the crop is hidden, not deleted — it stays inside the file, and anyone with a PDF editor can bring it back. Never use cropping to remove sensitive information; that job is called redaction, and this is not it. The tool repeats this warning after every crop, on purpose.