A contract, a tender or a set of minutes without page numbers is a document nobody can refer to in a meeting. Word puts numbers in before it exports; a PDF that arrived from somewhere else has no such luxury. This adds them to the file itself.
Free on every plan, including the free account.
Drop the PDF onto the page. It is read by your own browser and goes no further.
Bottom centre is the convention for a report; a corner suits a document that will be bound or hole-punched. Every page is numbered in order.
The new PDF downloads straight to your device with the numbers written into it, so they are there for whoever opens it next.