Open the PDF in a PDF editor that supports page numbering
Select the page numbering or header and footer tool
Choose the page position, such as top or bottom of the page
Set the alignment, such as left, center, or right
Choose the page number format, such as 1, 2, 3 or i, ii, iii
Set the starting page number if needed
Apply the page numbers to all pages or selected pages
Save the updated PDF file