Just right-click on the document in the document store of the specific Case and select "Send via Outlook" in the context menu that opens.

If you want to send multiple documents from a single Case, you can select these documents sequentially with the mouse and hold the "ctrl" key for WIN or "cmd" key for IOS during this operation. Once selected, left-click and you will again see a menu with the option "Send via Outlook".

Then the Outlook app opens and you continue as you are used to.