Rotating fixes pages that open sideways or upside down, usually because a scanner fed them in the wrong orientation or a phone camera recorded a rotation the PDF writer ignored. The change is written into the file itself rather than applied in a viewer, so the page opens correctly everywhere afterwards: in a browser, in Acrobat, on a phone, and — the one that matters most — on a printer. You can turn the whole document at once or name specific pages, which is the common case for a scan where one sheet went through rotated. Rotation is stored as a property of the page rather than by redrawing anything, so nothing is re-encoded and no quality is lost, and the file size barely changes. Everything runs in your browser; the file is never uploaded.
Why a viewer rotation does not stick
Most PDF readers let you turn the view, and that setting usually lives in the reader rather than the file. Send the document on and the recipient sees it exactly as it was, which is why a page that looks fine on your screen arrives sideways in someone else's inbox.
Writing the rotation into the file changes the page's own orientation property, so every reader honours it. That is what happens here, and it is the difference between a document that looks fixed and one that is fixed.
Rotation costs nothing in quality
A PDF page carries a rotation value of 0, 90, 180 or 270 degrees, and changing it tells the renderer how to present the existing content. Nothing is re-drawn, re-compressed or resampled — scanned page images stay bit-for-bit identical, and text stays selectable in the same reading order.
That also means the file size is effectively unchanged and the operation is instant regardless of document length. Arbitrary angles are not possible: PDF only defines quarter turns, and anything else would require re-rendering the page as an image.
Fixing a mixed-orientation scan
Batch scanners routinely produce documents where most pages are upright and a handful are not, particularly where landscape tables or a stapled insert went through the feeder differently. Rotating everything would fix those and break the rest.
Name the affected pages instead. Work from the thumbnails, note the positions, and apply the turn only to those — then check the result before sending it, because a page rotated 180 rather than 90 looks obviously wrong on screen and is easy to correct while the file is still open.