A reliable numbering plan separates source-page identity from the text drawn on that page. Source page 2 may display 1; source page 12 may display 11. Write down both endpoints before processing.
Define four visible facts
Record first source page, last source page, first visible number and format. The final number is first number plus selected page count minus one. Then choose a position with enough clear margin on every selected page.
A new PDF needs an independent check
PDFNumber draws visible text and serializes a separate file. It does not promise byte identity or preservation of every document feature. Keep the untouched source, open the download in another trusted viewer and inspect every numbered page, overlap, contrast, form, link, annotation, accessibility relationship and signature before relying on it. Compare the first, a middle and the final selected page, plus one untouched neighbor on each side when available. Confirm the page count and order, reopen the saved file rather than the browser preview, and test the viewer or printing workflow that will actually receive it.