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.
No. It changes a stored value and leaves the page content untouched.
Yes, rotate each one you want individually.
No.