With Showcase, you have the ability to dictate whether your shareable files can be shared from anywhere in your showcase or shareable only from specific areas within your showcase (more on that here). The content you put into these lists do NOT have to already be loaded into your showcase - you are able to load files directly to these lists, without needing them in your showcase.
If you want files to be shared from anywhere within a showcase - here's how you go about it:
1. In the Home tab of the website, click the ‘Manage sharing’ link associated with the showcase you wish to add shared content to. You can also click 'Manage Sharing' when you're in the editor too.
3. In the pop up, you will see two sections: Shareable from this slide and Shareable from all slides. To add content to Shareable from all slides - click Add File.
4. This will pull up the file picker where you can either select a file from your file library, or upload a new file.
5. Once you've uploaded your file, you're good to go! You'll see the file name loaded under the Shareable from all slides section and you can easily remove a file again by clickikng the small 'x' on the file name.
Publish your changes so they take effect! Once you've published, your file will appear in the sharing section when your users tap the sharing arrow in the right hand corner of the app: