Rotation that survives sharing
The frustrating thing about a sideways PDF is that it often looks fine on your screen. Your viewer rotated it for display, you sent it off, and the recipient opened a document lying on its side. The view rotation lived in your app, not in the file.
This tool changes the page orientation stored inside the PDF itself. Every page is turned by the angle you choose, the change is permanent, and the file opens correctly in Adobe Reader, macOS Preview, browsers, phones and print dialogs alike. Because only orientation metadata changes, the operation is instant and the quality and size of the document stay identical.
Scans, faxes and photographed documents
The usual suspects are batch scans fed in landscape, double-sided documents where every second page ends up upside down, and phone photos taken with the paper turned. Rotate the file here, then continue the cleanup: split off the pages you need, merge multiple corrected scans into one document, or compress the result if the scanner produced a huge file.