Why pages end up sideways in the first place
Rotation problems almost always originate at the scanner or the camera. A document fed into an automatic document feeder in landscape produces landscape pages regardless of how the text runs. A phone photograph carries an orientation flag that some software honours and other software ignores, which is why the same file can look correct in one viewer and rotated in another.
The other common source is a mixed document. Somebody scans a stack in which one page — usually a spreadsheet, a plan or a signature page — was fed in the other way round. The result is a PDF that reads normally until you hit a page that requires turning your head, and if you are going to send it to anyone, it is worth two minutes to fix.
Rotation is stored, not redrawn
A PDF page carries a rotation property that says how the page should be displayed: 0, 90, 180 or 270 degrees. Rotating a page sets that value. The page content itself is untouched, which is why rotation is completely lossless — nothing is re-rendered, no image is re-encoded, text stays selectable, and rotating four times returns you exactly to where you began.
This also explains a behaviour that confuses people. If a viewer appears to ignore your rotation, it is usually showing the page in its stored orientation rather than the requested one, or you are looking at a cached preview. Reopening the file, or opening it in a different reader, normally resolves the discrepancy. The rotation is genuinely written into the file, not applied as a temporary view setting.
Rotating some pages and not others
The common real-world case is not a document that is uniformly wrong but one where a handful of pages need turning. Rotate those individually rather than rotating everything and correcting the rest afterwards — fewer operations means fewer chances to end up with a document that is wrong in a new way.
Direction is worth pausing over for a second, because getting it backwards is the single most common mistake. If the text runs bottom-to-top up the left edge of the page, the page needs turning clockwise. If it runs top-to-bottom down the right edge, it needs turning anticlockwise. Upside-down pages need 180 degrees and the direction is irrelevant.
Rotation is also worth doing before you print rather than relying on the print dialogue. Print-time rotation is not saved, so the next person to open the file hits exactly the problem you just worked around.
Rotation, printing and how other people will see it
A rotated page prints the way it is displayed, which is usually the point of rotating it. What catches people out is paper orientation: a page rotated to landscape still prints onto portrait paper by default, so it arrives shrunk into the middle of the sheet with wide margins above and below. If the page matters, set the printer to landscape for it, or accept the smaller result deliberately rather than by accident.
It is also worth knowing that not every system honours page rotation equally. Well-behaved readers respect it, but some older document management systems, some email preview panes and a few automated processing pipelines read the raw page content and ignore the rotation flag entirely. If you are submitting to a portal that has rejected sideways documents before, it is worth checking how the preview renders rather than assuming.
For anything going to a person rather than a system, rotate and then reopen the finished file before you send it. Ten seconds of checking catches the one page you missed, and a document that arrives correctly oriented is one fewer reason for someone to send it back.