If the PDF is already split up into page numbers and they show correctly in Acrobat reader, then you can create a calculated field in the table window using the expression page().
If you need Monarch to work out the page numbers for you from a PDF that isn’t broken up nicely, then create a ‘Page Header’ template on the line(s) that identify a new page. Then use the calculated field as above.
Steve's quite right, and Page() or a Page Header template should cover nearly all cases.
However, some PDF files have more than 254 lines of text on a page, and if there's no clear way to define a page header, then you might a snag.
Hopefully a future version of Monarch will allow for this exception by removing or increasing the 254 line limit.