PDF toolsMost PDF readers, including Preview on Mac, Adobe Reader and the PDF viewer built into Chrome, let you rotate a page's on-screen view with a toolbar button or a keyboard shortcut. That rotation is often a display setting only: it changes how that page looks in that specific window, in that specific app, right now. Close the file and reopen it, or open it in a different app, or attach it to an email, and it can snap back to the orientation stored in the file, because the file itself was never changed.
View rotation vs. page rotation
A PDF page stores an actual rotation value as part of its data: 0°, 90°, 180° or 270°. A reader's rotate button sometimes writes that value permanently when you save, and sometimes just rotates the display temporarily until you tell it to save in a specific way, like File → Export rather than a plain save, and the exact behaviour varies by app and by version. That inconsistency is the whole source of the frustration: it's easy to rotate, look at a correctly-oriented page, and still end up with a file that reverts.
The reliable fix
Rotate PDF writes the rotation directly into the file's page data, the same value every reader respects, so there's no ambiguity between "looks right here" and "is actually right." Add the file, choose the rotation, and the download is correctly oriented everywhere: in a browser, in Preview, printed, or opened by someone else entirely.
Mixed orientations in one document
A scanned document often has a few pages fed in sideways while the rest are upright, typically a landscape table or chart inside an otherwise portrait report. Rather than rotating the whole file and making the other pages wrong, rotate only the affected pages: the tool lets you apply a rotation to a chosen page range instead of the entire document, so a single landscape page can be fixed without touching the rest.
Questions
Why does my PDF look right in Preview but sideways when I email it?
Preview's rotate command doesn't always rewrite the file's underlying orientation value on a plain save; it can be a per-window display setting. The recipient's app reads the file's stored data, not your screen, so it shows the original orientation.
Does rotating lose any quality?
No. The pages themselves aren't redrawn, only the orientation value is changed, so text stays sharp and selectable and images are untouched.
Can I rotate just one page out of a 50-page PDF?
Yes, specific pages or ranges can be rotated independently of the rest of the document.
Will this fix a PDF that's permanently rotated the wrong way from a bad scan?
Yes, that's exactly this case: the scanner wrote the wrong orientation into the file, and rotating it here writes the correct one in permanently.