Why scans end up sideways
A landscape table scanned on a feeder, a phone photo taken with the screen lock on, or a document placed upside down on the glass all produce pages that open rotated. Most PDF viewers let you turn the view, but that is temporary. The next person who opens the file sees it sideways again.
Rotating here changes the page setting inside the file, so the correction is permanent.
Another cause is a document whose pages were created in landscape orientation but saved with the wrong flag. Rotating the affected pages sets them straight, and you can rotate others the opposite way if a stack has mixed directions.
Choosing the angle
Ninety degrees turns the page clockwise by a quarter turn. A 270 degree rotation is the same as a quarter turn counter-clockwise. Use 180 degrees for a page that is upside down. If you are not sure, apply one angle and check the preview, then adjust.
For a mixed document, say a few landscape tables inside a portrait report, select just those pages and rotate them alone. Everything else stays as it was.
Use the thumbnails or the first page preview to judge the direction. A page that has its top edge on the left needs a 90 degree turn clockwise. A page with the top edge on the right needs 270.
Rotation versus changing the content
The tool sets a rotation flag on each page rather than redrawing it, so there is no quality loss. Text remains selectable and the file size barely changes. Some very old viewers ignore the flag, but virtually all current software, including browsers and phone readers, respects it.
If you also need to reorder or delete pages, do that in the organize PDF tool afterward.
Viewers that show a page differently from others are usually reading an older rotation flag from the source file, which this tool replaces.
Practical notes
The PDF may be up to 100 MB, and your file is not uploaded at any point. Encrypted PDFs are not supported. After saving, open the new file once and flip through to confirm every page faces the right direction before you send it.
If you need to rotate and then reorder or trim the same file, do the rotation first. Then open the result in the organize PDF tool, where the thumbnails will already show every page the right way up.
Rotation is also handy before you print. A landscape page that comes out sideways wastes paper and ink. Fix it once here, and every future print of this file comes out right.