Can I rotate just one page instead of the whole document?
Yes. Each page thumbnail has its own rotate button, so you can fix a single sideways page without affecting the rest of the document.
Does reordering pages modify the original file?
No. Your original PDF stays untouched — reordering, rotating, and deleting only apply to a new PDF that is generated when you click Save.
Is there a page limit?
No hard limit — very large PDFs (100+ pages) may just take a bit longer to generate thumbnails, since everything renders locally in your browser.