When you send a document for review in Salesforce Lightning, you must disable clickjack protection (if enabled) to view Visualforce pages in Lightning.

To disable Clickjack Protection

  1. Go to Setup > Security > Session Settings.
  2. Unselect:
    • Enable clickjack protection for customer Visualforce pages with standard headers
    • Enable clickjack protection for customer Visualforce pages with headers disabled
  3. Click Save.