I'm not sure my colleague has understood your question, do you mean you want to provide download after a user paid for it through Paypal?
This is not implemented into Dropfiles. Though, you can do it by combining Dropfiles to other extensions, like Payplans(https://stackideas.com/payplans) or any other ecommerce extension.
The idea is to set the downloads you want to be paid as private and only available for a user group. Configure your ecommerce so that when the user paid an item it is automatically set into the group that has access to the download.