Skipping a cover is a range decision, not a hidden offset. For a 12-page source with one cover, select source pages 2–12 and start at 1. The preview should show 11 labels ending at 11.
Check both boundaries
After download, confirm the cover has no added label, source page 2 shows 1 and source page 12 shows 11. Also check a middle page so a correct endpoint does not hide overlap elsewhere.
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.