Page numbers are generated automatically in PDF outputs. In most themes, front matter will be given lower roman numeral page numbers (according to traditional book design practice), and the Main Body will have arabic numbers. Different themes will display page numbers in different ways.
Reflowable EPUBs don’t have fixed page numbers in the same way as print books — if you change the font-size, or screen-size, pagination changes. So our EPUBs do not have pagination.
Some publishers embed a reference to print page numbers in their ebooks. Pressbooks cannot do this automatically, since the system doesn’t link an EPUB to a PDF — you might choose one theme for an EPUB and another for PDF, and Pressbooks has no way of connecting the two.