Why is my scanned PDF sideways?
The scanner saved the page image in one orientation and recorded a rotation flag telling readers to turn it. Readers that honour the flag show it straight; readers that ignore it, and most print dialogs, show it sideways. Rotating the page properly rewrites it so every reader agrees.
Two rotations, and they are not the same
A PDF page has a /Rotate value: 0, 90, 180 or 270. It is a viewing instruction, not a change to the page. The image underneath is untouched.
That is why a page can look correct in one application and wrong in another. It is also why "rotate view" in a reader does nothing on the next machine — you changed how you were looking, not the file.
Why scanners do this
Document feeders pull pages at whatever angle they enter. The scanner captures the image and marks the orientation separately, because rotating the bitmap costs time on the device.
Duplex scanning makes it worse: the back of a page is captured upside down relative to the front, so every second page needs a 180-degree flag. If the batch was mixed, some pages need nothing and some need 180.
The fix that sticks
- Open the file in a rotator that writes the change into the saved file, not just the view.
- Turn the affected pages, and check each one. Mixed batches rarely need a single global rotation.
- Download, then reopen the downloaded file. Confirm the pages are straight before you send it.
What to check before rotating
Rotating a page that carries a printed footer moves the footer too. If the document will be printed and bound, check that the margin you gain on one side is not the margin that was holding the binding.
If the page is also skewed — a slight tilt rather than a clean quarter turn — rotation will not fix it. That needs a crop or a rescan.
Where this stops
Rotation does not improve legibility. A page captured at an angle with a shadow across it is still that page after it is turned upright.
Does rotating reduce quality?
No. The page content is rewritten with a new rotation value and the image is not re-compressed.
Why does it revert when I reopen it?
Because you rotated the view, not the file. The reader kept the change in memory and the saved file still carries the old value. Use a rotator that exports a new file.
Can I rotate only some pages?
Yes. Apply the rotation per page — that is the normal case for duplex scans, where only every second page is upside down.
This page is general information, not advice for your document. The tools do what they say on their own pages, and every limit is printed there.