A watermark isn't a special PDF feature — it's just text, drawn onto the page like any other content, sized and angled to sit across whatever's already there without hiding it entirely. Making it read clearly at a diagonal, at low opacity, on every page regardless of size or orientation, is really just repeating the same drawing operation once per page with the same settings.
Because the stamp is drawn directly onto the page content, it opens identically in any PDF reader, and it's genuinely part of the file from that point on — not a removable layer or annotation.
Everything runs in your browser. Your file is never uploaded, never queued on a server, and never deleted from one — because it was never there.