Why rotating often does not stick
Many PDF readers only rotate the view. In Adobe Acrobat Reader, for example, View → Rotate View turns the page on your screen but does not change the file, so it opens sideways again next time and for everyone you send it to.
In your browser
- Open Rotate PDF and drop in the file.
- Choose 90° clockwise, 180° or 90° counter-clockwise, for all pages or only the pages you list.
- Click Rotate and download the new PDF. The rotation is saved in the file.
Different pages need different turns
Organize PDF Pages shows every page as a thumbnail with its own rotate button, so a scan with a few sideways pages can be fixed in one go.
On a Mac: Preview
Select the pages in the thumbnail sidebar, click the rotate button in the toolbar and save the file (File → Save). Preview writes the rotation into the PDF.