Turn pages that are sideways or upside down so the document reads properly on screen and prints the right way up.
Most scanners store the sheet orientation in the page dictionary and some viewers ignore it, so a page can look upright in one program and sideways in another. Setting the rotation explicitly fixes the file itself, not the viewer, which is why it also fixes printing and thumbnails.
For a document that was scanned landscape but should be portrait, rotate every page by 90 degrees. For a single folded insert, select just its pages. Two rotations of 90 are allowed; a 45-degree rotation is not, because pages are rectangles and PDF has no concept of a diagonal page.
Rotate, download, and open the file in a different program than the one you used before. If it now reads correctly in both, the file is genuinely fixed.
Yes — give the page number instead of applying the rotation to all pages.
No. Only the page orientation changes; the content is untouched.
PDF pages are rectangles with a rotation attribute limited to quarter turns; an arbitrary angle would have to be baked into the content, which changes the document.