To link a .pdf to an image, you will first need to upload the.pdf file to your documents library. Your documents library is accessed from the Campaigns page. Once you've uploaded the .pdf, click on the small downward arrow on the right-hand side of the page and select "Copy link". This will copy the url of the .pdf file to your clipboard, which you can then paste into the URL field for the image in your campaign. You'll want make sure you have Flash enabled by default on Emma URLs so that the link is copied successfully.
I hope this information helps! You can read more about adding links here!