How to rotate a PDF permanently (not just the view)
In short
The rotate button in most PDF readers only changes how you see the document right now, on your screen. For the rotation to stay for the recipient too, rotate the pages in the tool below and download the new PDF.
Do it right here
Rotating the view vs saving the rotation
PDF readers such as Adobe Acrobat Reader or your browser's viewer usually have a button to rotate the view. It doesn't change the file: when you close it or send it, the pages look as they did before.
A saved rotation changes the orientation of the pages in the file itself. That's what the tool on this page does: you get a new PDF and the original stays unchanged.
Only some pages are sideways
Scans often have just one page sideways, such as a landscape appendix. In the tool, each page has its own rotate buttons, so you only fix the wrong ones.
After rotating
If the document is going to a portal, check its size too. Rotated scans can stay large; shrink them with Compress PDF.