Official response from Gavin DeSchutter completed
If you just need to link it for download, you can do that in the files section and then hyperlink to the file on the public page –http://nationbuilder.com/how_can_i_link_to_a_stored_file_on_my_pages
If you are trying to display a PDF map, you can post it on a PDF display software like Scribd, then post the URL (not hyperlinked) to the PDF display in your content editor – the Embed.ly integration in NationBuilder will convert the URL to a display.
Official response from Jesse Haff
You can change the credit card by logging into your control panel and clicking Settings > Billing & Addons > Billing Info and adding the new credit card details.