It is one of the most common small PDF annoyances. A scanned page is sideways, you press the rotate button in your reader, it looks right, you send it, and the person at the other end replies that page three is still on its side. Or you open the file again tomorrow and it has flipped back. The rotate button did work. It just did not do what you thought.
Rotating the view vs rotating the page
Most PDF readers, including the ones built into browsers and Adobe Acrobat Reader, have a rotate button that turns your view of the document. It is like turning a printed page on your desk: the page itself has not changed, and nobody else's copy turns with it. When you close the file, the view resets.
Rotating the page means changing the file. Every PDF page carries a rotation setting, in steps of 90 degrees, that tells every reader how to display it. Change that setting, save the file, and the page shows the right way up for everyone.
Rotate PDF changes the rotation stored in the file, for all pages or selected ones, so the fix travels with the document. For a document with a mix of correct and sideways pages, Organise PDF shows every page as a thumbnail and lets you turn each one individually.
Why phone scans come out sideways
Phone cameras capture every image in the same orientation, whichever way you hold the phone, and attach a small note saying "display this rotated 90 degrees". Photo apps read the note, so the picture looks right in your gallery. Some tools that turn photos into PDFs ignore the note and place the image as it was captured, which produces sideways pages.
The JPG to PDF tool on this site reads that note and turns each photo the right way before placing it, so phone photos come out upright. If you are using another tool and get sideways pages, fix them afterwards with Rotate PDF.
Landscape pages are not wrong
Not every sideways-looking page needs fixing. A wide table or a chart is often deliberately set on a landscape page. In a bound report that page is turned so the reader rotates the document clockwise to read it. On screen, rotating it to read naturally is fine; in a document going to print, check with whoever is printing it before changing it.
If an upload portal or printer wants every page portrait, Resize pages with Always portrait fits landscape pages onto portrait sheets without rotating them.
Does rotating reduce quality?
No. Rotating by 90 degree steps only changes the page's rotation setting. Text, images and drawings are not re-drawn, so the file keeps exactly the quality it had and barely changes in size. Slightly crooked scans are a different matter: straightening a page tilted by a few degrees means redrawing the image, which a rescan does better than any tool.