Ok, I think I might have this figured out now...but please don't hate me if I am wrong.
You need to go to Extras > Media Types. Click Insert. Your type of file would be PDF and your extension would be .pdf Click save.
Then go to Extras > Media Manager. Click Insert. Type the name of your collection (such as E-BOOKS). Click Save.
Click Edit to select files to add to the collection. Browse, find your file, then click Add. Repeat this to keep adding to this particular collection.
Click on Extras > Media Manager. Choose your collection and click on Assign To Product. Choose the category from the drop box. Click Add.
Make sure you have the following settings:
In Configuration > Attribute Settings, make sure downloads are enabled. Choose true if running on a unix/ linux/ mac. Choose false if running on windows. Download by streaming - set to true if downloab by redirect is false or vice/ versa - depends on your PHP memory limit, the screen will explain.
In Modules > Shipping, set free shipping to active and disable all other options.
In Configuration > Stock, there is a place for "allow checkout", try this as true and false. I do not know which one you should use.
In Configuration > Product Listing, Display Product Quantity should be false.
Configuration > All Listing > Display Product Quantity - set to zero.
You can change other things to "turn off" store type features. You just have to play around. I advise you to take notes of everything that you change in case you have to go back and change something back.
Hope this has helped. And if not, my apologies for wasting time. Have fun! :D