Rotation is the smallest job in this set and it has one real failure mode, which is doing it in the wrong place. A PDF reader can turn a page on screen without touching the document, and several tools that advertise rotation do exactly that: the page looks right in front of you, you download it, and the person you send it to gets it sideways again. Worse, so does the printer.
This writes the orientation into the page. A landscape page is landscape in the file, so it opens correctly in Acrobat, in Preview, in a browser, on a phone, and comes out of a printer the way you saw it.
The common cause is a scanner. Feed a stack through and one sheet goes in the wrong way, so a twelve-page document has eleven correct pages and one that needs turning. That is why the tool works on a selection rather than on everything: pick the page that is wrong, turn it, done. If the whole document came out sideways — which happens when the original was landscape and the scanner assumed portrait — select all and turn them together.
Rotating sits with the other page operations, so it is an ordinary edit. It steps back one at a time, which is different from the whole-file tools like compressing or numbering where the change rewrites everything and the escape is starting again from the file you opened. That distinction is worth knowing because it changes how freely you can experiment.
Two things it does not do, stated so you are not looking for them. It turns in ninety-degree steps, so a scan that is crooked by three degrees cannot be straightened here. And it does not change page size — a portrait page turned sideways becomes landscape because its dimensions swap, but nothing is stretched, scaled or cropped.
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