One possible solution would be to use the Open Image button on the Button Bar.
The Open Image button will open the image's linkURL (in a tab specified by the corresponding linkTarget).
(If an image does not have a linkURL entry, then the Open Image button will use the imageURL instead.)

Therefore, you could add linkURL entries to your gallery pointing towards the files that
The linkURL entries can be relative (to the page containing the gallery's embedding code) or absolute (in the form 'http://www.example.com/images/image0001.tiff').

If using JuiceboxBuilder-Pro to create or edit your galleries, you can add linkURL entries on the 'Images' tab. (Just click a thumbnail to reveal the input fields.)

If you like, you could change the Open Image button's icon (following the Using Custom Icons support section in the Theming Guide) and change its rollover tooltip text using the languageList configuration option. (Please see the International Gallery Text support section for details.)

Alternatively, you could just set up links in your image captions to link to your custom files.
You can add links to captions using HTML formatting (specifically the <a> tag) as noted in this FAQ:
How do I add HTML formatting to image captions and titles?

If using the caption suggestion above, you could even use the download attribute in your caption link (supported by Chrome, Edge, Firefox and Opera) to instruct the browser to download your custom file instead of navigating to it.
A caption link (entered via JuiceboxBuilder-Pro's 'Images' tab) might look something like this: