While preparing your send out and under the Engage Tab you will see an Identify New Viewers as a feature to enable. This will give you full vision of who opens the send out once delivered and is great when there are multiple decision makers involved.
For example, if you send the document to a Director of Sales, but she forwards the email to her VP of Sales (using a different IP Address), you will know the VP of Sales' name and email. This helps you begin to map your customers' buying journeys and creating best practices to address them.
The great thing about the pop-up is that is will appear as if the original recipient is doing her due diligence and granting security access to the new recipient. Once they authenticate themselves with a name and email, you will receive their contact information in real time and know how long they are looking at the send out.