Numbers where readers expect them
Reports, theses, contracts and merged documents are much easier to discuss when every page has a number. Bottom center is the classic choice; bottom right suits documents that will be printed and bound on the left. The Page 1 of 10 format makes it obvious when a page is missing.
Use Start at when the PDF is part of a larger document, for example a chapter that begins on page 37, and turn on Skip first page to leave a cover or title page clean.
Clean, selectable numbers
Numbers are added as real text in a neutral gray, small and unobtrusive. Rotated pages are handled correctly, so the number always appears upright at the edge you chose. Your files are processed in the browser and never uploaded.