Numbers go at the bottom centre of every page in small grey type — visible when you look for it, quiet when you do not. You can start counting from any number and skip the first page when it is a cover. Pages stored sideways by a scanner are measured, and the number is placed so it reads upright on the screen instead of running along the edge.
The PDF engine (about 500 KB) loads only when you drop a file in, so the page itself stays light. Your file is read on your own machine and never sent anywhere — open your browser's network tab and watch while you use it.
At the bottom centre of every page, in grey 10 point type, 26 points up from the edge. It is drawn over the page content, so if a footer already sits in that spot the two will overlap — the existing footer cannot be pushed aside.
The number still comes out at the bottom centre of what you see, reading horizontally. Scanners often store pages sideways with a rotation flag, and most tools stamp the number as if that flag were not there, which leaves it running up the side of the page. Here the rotation is read and the position adjusted for it.
Yes. Set the starting number and the count carries on from there — useful when the file is one chapter of a longer document. Anything below one is treated as one.
The starting number. Skipping the cover does not use a number up — the first page that is stamped receives the number you chose to start from, and the count runs on from there. With the defaults, the second page of the file carries the number one.
These all work the same way — in your browser. Most are free; the ones marked as needing a plan are not.