Rotating the viewer is temporary; rotating the PDF page changes how the document opens for everyone. Use page-level rotation when only part of a scan is sideways.
The safest interface shows every page and its final orientation before saving.
Select precisely
Upload the PDF and inspect the page grid. Select only the affected pages, then rotate clockwise or counter-clockwise in 90-degree steps. A 180-degree correction takes two quarter turns.
Watch for mixed page sizes
Portrait and landscape pages can legitimately coexist. Rotate a page because its content is wrong, not merely because its shape differs from the page beside it. Forms, maps and foldouts often need landscape orientation.
Verify the saved document
Download the result and reopen it in a separate viewer. Check page thumbnails as well as the full page, because cached browser previews can occasionally show the old orientation. Confirm interactive elements and form fields still align with their pages.