To use our service for that, you can either .zip the .mp3 and .pdf files together into a single file for each product, or you can add the .pdf as its own product, each .mp3 as its own product, and create an additional product for each .mp3 that bundles the .mp3 with the .pdf (so essentially the latter would create two products for each .mp3).
As a simpler and more economical alternative, you may wish to simply upload the .pdf file to your own Web site, then add a static HTML link to the .pdf's URL in your thank-you/download pages:
http://www.e-junkie.com/ej/help.custom.thankyou-page.htm