How does cropping work?

Set top/bottom/left/right margins as a percentage and they're trimmed from every page's visible area at once.

Is the cropped content gone forever?

No — cropping sets the page's crop box, which viewers respect but which can be un-cropped later. For permanent removal use Redact in the PDF editor.

Never — the crop happens on your device.

Cut scanner borders and whitespace off all pages of a PDF with per-side margin control. Private, in your browser.