Turn pages the right way up and keep it that way. Unlike rotating in a viewer, the rotation is written into the file, so it opens correctly for everyone.
How to Rotate PDF Pages
Open the PDF — Drop in the document with sideways pages.
Turn the pages — Rotate everything, or only the pages that need it.
Download — The saved file keeps the new orientation.
Questions
Why does my PDF go back to sideways after I rotate it?
Most viewers rotate the view only and do not change the file. Rotating here writes the orientation into the document, so it stays put wherever it is opened next.