I have a pdf document I created through non-Acrobat means (printing to pdf, then merging a bunch of pdfs), but I'd like to manually change the page numbers (i.e. the first several pages are simply title pages, the page that is labeled "page 1" is really the 7th sheet of the pdf). What's the simplest (and ideally, free) way to do this?

To be clear, I am not trying to change the numbers on the pages themselves, but the page numbers in the "metadata" that the pdf stores (the pages themselves are already numbered correctly; I just want "go to page 1" to go to the page labeled 1, which could be sheet 7).

For what it's worth, I'm on Windows, though I have access to Macs as well.