It happens all the time with scanners and photos of documents: the file opens sideways and every time you reopen it you have to rotate it by hand in the viewer. Here the rotation is written into the document, so it sticks for whoever receives it too.
You can rotate every page in one go by leaving the pages field empty, or straighten only the crooked ones by entering their numbers. The rotation adds to any that is already there, so a page already turned by 90 degrees and rotated by another 90 ends up at 180.
The operation doesn't touch the content: there is no loss of quality, because only the orientation in which the page is drawn changes.