Short answer: drop the PDF, pick a corner, a starting number and whether the total shows, then download. The digits are real page text, so they print and copy with the page. Nothing is uploaded and the page count never changes.
What a submission usually wants
Court filings, college assignments and grant applications tend to ask for bottom-centre numbering with the total shown, starting at 1. A book-style document wants the cover unnumbered and the first content page to be 1, which is why the start number is an input rather than a fixed value.
Numbering after a merge, not before
If several files become one document, number the merged result. Numbering each part first gives you three pages numbered 1 and a document that reads as a mistake. Merge, then number, then stamp a watermark if it needs one — each step is a separate tool on purpose.