Why some PDFs open sideways
Scanners and phone-camera apps often save pages with a wrong rotation flag. The page content is fine — it's just the metadata telling viewers how to display it that's off. Rotating with this tool corrects the flag rather than re-rendering the pixels, so the result stays crisp and the file size barely changes.
Whole-document vs per-page
The bulk “rotate all” buttons are useful when an entire PDF was scanned the wrong way. For mixed orientations — a portrait report with a single landscape table page in the middle, for example — click the individual page thumbnails to set their rotation. Each click adds 90° clockwise.
Saves the rotation flag, not pixels
Some online tools rotate by re-rendering each page as an image — fast but it destroys selectable text. This tool sets the PDF's per-page rotation property and re-saves the file, which means the underlying text remains searchable and copy-pastable.