Documents within a bundle can only be downloaded once the bundle is completed by all signers. Before sending a bundle to signers you have the ability to print the uncompleted documents if you wish, learn more on how to do that here.
To download completed documents:
Go to your dashboard and click on the completed bundle you want to download.
Then, on the right side of the bundle popup you will see "Download PDF".
Note: Sometimes it will take a few minutes for the "Download PDF" to show up after a bundle has been completed by all signers.