A PDF page carries its own rotation setting separately from its content — a scanner that fed a page in sideways doesn't rotate the pixels, it just stores the page as-is and expects the reader to turn it 90° on display. Most readers get this right, but plenty of scans are saved without that setting correct, so the page opens sideways everywhere. Rotating here changes that setting directly, for every page in the file at once.
Because nothing about the page's content is touched, a rotated file is exactly as sharp and exactly as searchable as the original — just facing the right way.
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.