Two ways to split
Extract builds a single new PDF containing just the pages you list, in the order you list them — so "5, 1-2" produces page 5 followed by pages 1 and 2, which doubles as a page reorderer. Split every page produces a zip with one PDF per page, named by page number. Both run locally with pdf-lib; the only limit is your device's memory, and password-protected files need unlocking first.