"Download All" lets the end-user download a zip-package of all content in any particular media-content type that has 5 or more assets available on the content page.
a) If you have 5 or more Images uploaded to your content page, the end-user then can download all images at once.
b) 7 video and 4 images uploaded to your content page means only video can be downloaded at once.
To enable media-content download go to your Content Page settings > General Options > Download enabled or follow these steps:
1.) Click on Projects in the Navigation Menu.
2.) Open the Project where the Content Page, which you'd like to enable downloads, is in.
3.) Hover with you mouse over the desired Content Page, and click edit.
You are now at your Content Page's settings.
4.) Scroll down in General Settings until you find "Download" enabled.
Hint: The Download all button only appears if you have 5 more files uploaded for each media-type.
5.) After finishing the desired changes, click Save & Exit.
Hint: You will see a green message popping up on top if everything has been done properly.