Page numbers turn a stack of pages into a document people can find their way around: "see page 12". Reports, theses and legal filings are expected to have them, and you can add them without the original file.
How to add page numbers
Open Page numbers and drop your PDF on the page.
Pick where the number goes: any corner, the middle of an edge, or bottom centre like most books.
Pick the style (3, Page 3, or 3 of 12) and the size.
To leave the cover unnumbered, tick Skip the first page. Numbering then starts at 1 on page 2.
Click Add page numbers and download the file.
Which style?
A plain number is the most common. "3 of 12" tells the reader how much is left, which helps in long documents, and on printed contracts it shows that no page is missing.