The classic: rotated and yet not rotated
A scan comes out sideways, you turn it upright in the viewer, close the file, open it again later and everything sits exactly as before. That is not a bug, it is the difference between two things that look identical.
Rotating in a viewer changes only the display on your screen. The document itself stays untouched. Pass it on and you pass on the old orientation. Only when the rotation is written into the file does everyone see it the way you intended.
Rotating for good
That is what Rotate PDF does. Pages turn by 90, 180 or 270 degrees and the new orientation is saved into the document. Whoever opens it next sees the right thing straight away, in any program.
Processing happens in the browser. The file is not uploaded, which is welcome with contracts and official post.
When only some pages are wrong
The most common case when scanning with a feeder: some sheets went in rotated, others did not. Half the document ends up upside down.
Rotating the whole file does not help there. Instead you click each page and set it individually. The preview shows exactly which page you are working on.
Portrait and landscape on purpose
Some documents mix both deliberately: the report in portrait, the wide table in landscape. That is not an error and should stay. So rather than turning every page at once, only touch the ones genuinely wrong.
What usually comes next
Once orientation is sorted, the order often turns out wrong too, especially if the feeder pulled sheets the wrong way round. Organize pages fixes that by drag and drop.
Left-over blank pages from duplex scanning go with Delete pages. And if the margin is too wide because the sheet sat crooked, Crop PDF trims it away.
In short
Rotating in a viewer is only a view, saving makes it permanent. Individual pages can be set separately, and mixed formats may stay mixed. Order, blank pages and margins usually follow.