With FileEvent in Salesforce Real-Time Event Monitoring, you now get real-time notifications when a user downloads a file. File Event supports transaction security policies.
Where: This change applies to Lightning Experience and Salesforce Classic in Enterprise, Performance, Unlimited, and Developer editions where Event Monitoring is enabled.
Who: This change is available to customers who purchased Salesforce Shield or Salesforce Event Monitoring add-on subscriptions.
Track File Downloads in Near Real-Time with FileEvent (Beta) (salesforce.com)