Your file is processed entirely in your browser and never uploaded: it is read into memory on your own device, the rotation value stored on each selected page is changed, and the result is offered back as a download. Nothing reaches a server, and the tool works offline once loaded. Rotation here is a metadata change, not a re-render: a PDF page carries a stored rotation in multiples of 90 degrees, and this tool adds your chosen angle to that value. Page content — text, vectors, images — is not touched, redrawn or re-encoded, so there is no quality change of any kind.
Selection is by page range, stated honestly: you name the pages as ranges (for example “2-4” for a sideways scan in the middle of a document, or the whole file), and the chosen angle applies to every page in those ranges. This tool does not render page thumbnails for picking pages visually — the page count and the ranges are the interface, so it helps to know from your PDF reader which pages are sideways. Rotating by 90 or 270 degrees swaps a page's displayed width and height; 180 leaves the dimensions unchanged and turns the page upside down.
The change is permanent in the downloaded file because the stored rotation itself is rewritten — every reader will show the page the new way up, not just this browser. That is the difference from the rotate button inside a PDF viewer, which typically turns the view for the session only and leaves the file unchanged. If a page was already stored at 90 degrees and you add 90, the result is 180: rotations add, modulo 360.