How to download all items (filtered and unfiltered) in a feed

Go to Manage Feed > Advanced Settings
Change the Product Filters setting to Include products matching the filters. (If it is already set to that, change it to Exclude products matching the filters instead.)
Go to Manage Feed > Product Filters
Add a temporary filter that matches all the items, like id does not equal 'xyz'
Go to Manage Feed
Click the Refresh button.
Return to Manage Feed > Download Data Feed File when the task completes.
Click the Download button.
After downloading the file, be sure to change the Product Filters setting back to its original value, and remove the temporary filter.