Numbers are drawn at the bottom centre of every page in small gray Helvetica — the same discreet style on all pages, including the first. The “of Y” total is useful when reviewers need to know they have the complete document; the bare number keeps the footer minimal. Position and typeface are fixed rather than configurable: this is a quick, consistent stamp, not a layout editor. The whole pass happens locally with pdf-lib — the file stays on your machine and the result is always a new download, leaving the original untouched. The numbers are drawn as new text on each page, so they survive printing, copying and any later conversion of the document.
Printed reports, tender and legal bundles, academic drafts and any document that will be discussed over email all benefit: “see page 7” only works if pages are numbered. Number the file after the final order is settled — after Organize PDF if pages moved — because reordering a numbered document leaves labels that no longer match. If you need numbering anywhere other than the bottom centre (or a different font), build the page in HTML and print it to PDF instead.
Also useful: Watermark PDF, Organize PDF, and PDF Page Editor.