Hi,i just tried to add the pdf download file from localhost.Here are the steps that i did.
1. Upload the pdf file to the media tab menu file in the admin->catalog->products as shown in the 1st step.
2. Create downloadable file name in file summary as shown in 2nd and 3rd step.
3. On the 3rd step,select before order for the activated menu and set the status to on.
4.The downloadble file will be shown as on the achievement picture i've attached here.
Hope it helps.