Just right-click on the document in the document repository 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 down the "ctrl" key for WIN or "cmd" key for IOS. Once selected, left-click and you will again see a menu with the option "Send via Outlook".

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