How can I apply page numbers to the exported PDF?

First, you can insert a text using the "Text" tool under the Basic Form Elements.

After that, you can enable the "Show Headers and Text" option in the Submissions page. Please take note that if you enable this, all "Text" elements will appear in the exported PDF. Here's a guide: https://www.jotform.com/help/305-How-to-Show-Headers-and-Text-in-the-PDF.

Hope that helps. Thank you.

Thank you so much for this. My problem is that my forms will change size depending on the overall content. Are the page numbers fairly static?

Thank you in advance . . . 

Unfortunately, adding page numbers to the PDF report is currently not available and it is required to have them added manually. You may consider adding the text field to the PDF report by customizing it.

https://www.jotform.com/help/384-How-to-Customize-PDF-Submissions-Report

Please give it a try and let us know if you need any further assistance. We will be happy to assist.