There are several ways you can receive notifications regarding a document.

1. Subscribing to a document. This will notify you if anything is done to the document, whether it is a field update, a comment being added to the discussions, or the document making its way through a workflow process.

2. Email notifications via workflow. You can create a workflow that includes an Approval action where those taking the action have the Reviewer role. Once the reviewer is finished they can click approve or decline and an email notification can be sent to the desired parties to notify them that the document has been reviewed.

3. Mentioning a user in the Discussions panel. The reviewer can utilize the Discussions feature while reviewing the document, and mention those who need to be notified when they are done reviewing the document.

None of these solutions notify a user when a document is simply viewed, rather they require the reviewer to take some action, be it updating a field or adding to the discussions for option 1, approving or declining for option 2, or mentioning a user in the discussions for option 3.

You can find more information about each of the three methods by following the links below.

Subscribing to a document; https://nucleus.one/subscribe-to-documents/

Creating Workflows: https://nucleus.one/creating-workflows/

Using Discussions: https://nucleus.one/collaborating-using-discussions/