How rotation works
Each tap adds 90° to that page, cycling through 90°, 180°, 270° and back to its original orientation. Rotation is stored as a property of the page, so nothing is re-rendered and no quality is lost — the text stays selectable and the file size barely changes.
Scanned documents
Scanners often produce a mix of orientations in one file. Rotate the pages that need it, leave the rest alone, and download once — only the pages you touched are changed.