Drag a frame over the page and cut away white margins or parts you don't need. The same frame is applied proportionally to every page — all in your browser.
Browse the pages to see where the frame falls; it is applied proportionally to the chosen pages, all by default (rotated pages too).
This tool runs entirely in your browser: your PDF is not sent to a server. A PDF with a password opens here with that password, or you remove the protection with the Unlock tool; the password also stays in your browser.