Cropping trims the visible area of every page to a box you draw on the first-page preview. The usual reasons are scanner margins — the black borders and skewed edges a flatbed adds — and slide decks exported with generous whitespace that wastes half the screen when read on a phone. Drag the box once and it applies throughout, which works because documents are generally consistent: if page one has a two-centimetre dead margin, so does page forty. What actually changes is the page's crop box, the rectangle that tells a reader which part of the page to display. The content outside it is still in the file, just not shown, so cropping is a presentation change rather than a way of removing anything. It runs in your browser with nothing uploaded, and takes no toll on quality because nothing is re-rendered.
Cropping hides, it does not delete
A PDF page has both a media box, its full physical extent, and a crop box, the region a reader displays. Cropping sets the crop box, and everything outside it remains in the file — extractable by anyone who resets the boxes or runs the text extraction that ignores them.
That is fine for tidying margins and dangerous for anything else. If the material outside your box is sensitive rather than merely untidy, cropping is the wrong tool: redaction re-renders the page so the content is genuinely gone.
Why one box usually fits the whole document
Documents from a single source share a geometry. A scanned book has the same gutter on every page, a slide deck has the same margins on every slide, a printed report has the same header band. So a box drawn once from a representative page is normally correct throughout.
The exceptions are documents that were assembled from several sources — a merged bundle, or a scan where some pages went through rotated. Check a few pages at different depths before committing, because a box that suits page 1 can clip content on page 30 of a mixed document.
What cropping does for file size and reading
File size barely moves, because the content is still there. What improves is the reading experience: on a phone or an e-reader, removing dead margin means the text can fill the screen, which is often the difference between a scan being readable on a small display and not.
Printing benefits too, in a different way. A cropped page scaled to fit paper puts more ink on useful content and less on white space, so a book scan with the margins trimmed prints noticeably larger at the same paper size.