DPD now supports multiple file fulfillment per product, so you can attach any number of files to a product. When the customer purchases the product they will get a download link to each individual file you have uploaded.
To add multiple files to a Digital product in DPD, simply add more file fulfillments:
1. Log in to DPD
2. Go to Products in the left menu
3. Create a new product, or find an existing product in your product list
4. Click the product link to open up the Product Details page
5. On the details page, click "Manage Purchase Actions"
6. Add as many file fulfillments to the product as you wish: