Your PDF, its pages laid out in front of you. Drag them into the order they should have been in, drop the ones that shouldn't be there, and add pages — from another PDF, or a copy of one you already have, or a blank. Then save it back out as a PDF. Nothing printed on the pages is touched.
Page order is the one thing about a PDF that everybody needs to change and almost nobody has the software for. A scanner puts the last sheet first, an export lands the appendix before the summary, a signed page has to come out before the file goes to someone else. None of that is editing the document — it is moving whole pages around — and it should not need a subscription to a design suite.
This is the job that usually gets described as merging, and merging is the blunt version of it: two files, end to end, in the order you happened to upload them. What people actually want is nearly always narrower — page 4 of this one, in place of page 4 of that one. So a second PDF doesn't get appended and sealed: its pages arrive as cards alongside the ones already there, carrying the file they came from, and you keep the ones you want and drop the rest. Ten files can feed one document, and a page can come in more than once.
Pages keep their own size when they move between documents. An A4 page dropped into a Letter document stays A4, because resizing it would mean rescaling everything printed on it, and this tool doesn't touch what is printed on a page. Most readers and printers handle a mixed-size document; if yours has to be uniform, make the pages match before they come in here.
It doesn't touch what is printed on a page — no editing text, no redaction, no filling in forms. And it doesn't rotate pages: a page that arrived rotated stays rotated, and is drawn here the way a reader will show it.
Each page you keep is copied whole: its text, fonts, images, vector drawings and the annotations sitting on it, links included. The title and other metadata come from the document you started with — the one you are editing — rather than from whichever file happens to supply the first page.
Two things do not. Bookmarks — the outline down the side of a PDF reader — are a list of pointers into the page order of the file they belong to, and after pages have moved, been copied, been deleted or arrived from somewhere else, some of those pointers are simply wrong. A bookmark that quietly lands on the wrong page is worse than no bookmark, so the outline is dropped rather than guessed at. The same goes for the field catalogue of an interactive form: the widgets stay on their pages, but the form structure that binds them into a fillable document is tied to the arrangement they came from. If any of the files you use has either of them, this page says so before you save.
There is no PDF renderer behind this site — the libraries that rasterise a page are either copyleft-licensed or a native install, and neither belongs in a tool this small. So instead of a picture, each page card carries the things that actually identify a page: the first line or so of its text, its size in millimetres and the paper it matches, and a drawing of the page in its true proportions. A page with no text at all — a scan, a photograph, a full-page image — says so, which is itself worth knowing when you are looking for the scanned page in a document of typed ones.
A PDF that needs a password to be opened can't be read here, and won't be: there is nothing to show you. A PDF that opens for anyone but carries an owner password — the "do not edit" flag that every reader honours on trust — is read, and the copy you get back comes out without that flag. The page tells you when that has happened rather than doing it quietly.
Every file you upload — the one you're editing and any you take pages from — is read for its page list, used when you ask, and deleted within the hour by a sweep that runs whether or not you come back. Nobody opens it, and nothing out of it is stored — the usage log keeps the tool name, the file size, and nothing else. The text previews are built on the server and sent to your browser; they are not written down anywhere.