When you first downloaded your cases, you didn't download the associated content items; if you want to view them, you'll need to download them first. You can download content items from the case, person, or organization where they're connected.

First, navigate to the content item you want to download. You can use the Find... bar to search for a specific word or phrase, which will filter the list of content items by content type or comments.

Next, simply tap the content item. It'll download and appear for you to view.
​If the content item has a file type that is not supported, you won't be able to view it, but you can email it as well as manage its connections.