In the textfield element that you are using to collect the user's Email address, go to the Advanced tab. There you will see a field called "Mailbackfile." In that field, enter the full path to the file you want to send to the user, like this:

absolutePathToYourFileOnTheServer/images/referralcoupon.pdf

Click on the 'Save properties' button at the bottom of the screen to save the changes you just made to the element, then click on the SAVE icon in the upper-right hand corner of the screen to save the form itself. Then test it to see if it works.