As a point-of-interest:
You can get around the "digital media" requirement if you're willing to leverage your site's user roles -- something you'd probably only consider if you have a very limited number of PDFs to offer.  As we'll see in the example, it's fairly easily done but it also makes Catalook's integrated digital media features and $80 price look extra attractive...
 
Example:  
A module like the nifty $40 PayPal BuyNow from Gumbosoft has PayPal IPN support and can add users to user roles upon purchase (only after purchase is automatically verified via IPN).  If you offer file "PDF01.PDF" for sale, you'd create a security role "PDF01", create a file directory "PDF01" and limit the VIEW rights to user role "PDF01", upload file "PDF01.PDF" into directory "PDF01" and set your BuyNow module to offer the rights to user role "PDF01" for a fee.  A user buying "PDF01" actually buys the rights to the user role which, in turn, opens access to the file.  The user can return and access the file as often as needed without need to keep track of what the user has purchased.  Your "log" of who bought what and when is reflected by membership in the various user roles you set up for your PDFs. 
 
At only $40 this is certainly a cheap solution but, since it involves setting up a role and a directory for every PDF you wish to sell, it'd be a real headache to implement for more than a handful of PDF files.  If you DID decide to go that route, though, you'd probably also be interested in Secure Downloads (also from Gumbosoft) which will make life a little easier regarding file permissions management plus give you logging and control over how many times a user may download a file.  Then again, you could have simply bought Catalook for the same price as the two Gumbosoft modules and Catalook offers many, many more features...
 
 
Hope that helps-
-mamlin