Instead of choosing your action application, click on the + sign between where you choose the applications. This will give you the option to choose the "Only continue if..." filter.
You can choose different filters based on the Document that is being sent or the Recipient that is receiving the document. In our use case, I wanted to send the case study to the recipient, only if they have reviewed the Post-Meeting Notes I sent them.
To do this, I chose Only continue if... Document Name | Text Contains | Post-Meeting Notes. I plan on naming my post meeting notes: Post-Meeting Notes: Client Name so the filter will always find the event.
Click continue to add the action application.