Scanned pages often come out sideways or upside down: a sheet fed the wrong way, a phone held sideways. Turning them in a PDF viewer only changes your view, and the file opens wrong again for everyone else.
Rotating in a reader such as a browser or Acrobat lasts until you close it. To fix the file itself, so it prints the right way and opens correctly anywhere, the rotation has to be saved into the PDF. That is what Rotate pages does.
You rarely need to turn the whole document: turn only the two or three pages that came out wrong, and the rest stay as they are.
Do it now: Rotate PDF