So nobody has to tilt their head
You photographed the electric bill with your phone, saved it as a PDF, and it opens lying on its side. The kids' drawings scanned for their grandparents came out upside down. A report has every sheet upright except three plans that were fed in sideways. A PDF reader can turn the view, but usually only while you are looking – the person you send it to gets the same crooked file. Rotate PDF writes the new orientation into the document, so it stays after sending.
The whole document or just some pages
"Left" and "Right" act on all pages at once, a quarter turn per press. Clicking a single thumbnail turns only that page, always to the right: one click is 90°, two clicks 180° (upside down), three clicks 270°, and a fourth brings it back to where it started. Thumbnails turn immediately, so you see the result before saving.
If you change your mind
"Reset rotation" returns every page to the position it had when you opened the file. "Rotate PDF" only works once at least one page has been turned.
What changes in the file
The turn is stored as a page property and added to whatever orientation the page already had. The content is not redrawn, so text stays searchable and images stay as they were. You get a new file with "-rotated" added to its name, and the original is unchanged. Everything happens in your browser – the document is not uploaded.
Good to know
To move or delete pages as well, use Organize PDF. A password-locked file has to go through Unlock PDF first, if you know the password. Photos of documents become a PDF with JPG to PDF.