One landscape page in a portrait document, or one sheet that went through the scanner the wrong way. You only want to rotate that page.
A PDF page carries a rotation value alongside its content. Rotating a page changes that number rather than re-rendering anything, so nothing is re-compressed and no quality is lost, however many times you do it.
That is also why a page can look correct in one reader and sideways in another. Some software ignores the stored rotation. Saving a corrected file makes the orientation unambiguous everywhere.
A genuinely landscape page, like a wide table or a chart, is meant to be that way. Rotating it to match the rest of the document will make it unreadable, so check whether the page is wrong or simply different.
Find the page in the thumbnails and press its Turn button. Each press is a quarter turn, so three presses make a three quarter turn. The other pages are left exactly as they were, and you can see the new direction on the thumbnail straight away.
Save the PDF when every page looks right. The turn is stored in the page, so the file opens the right way in any viewer. The saved copy is named after the original with fixed added.
No. It changes a stored value and leaves the page content untouched.
Yes, rotate each one you want individually.
No.
Keep pressing its Turn button until it is upright again. Four presses bring it back to where it started.