You need to configure the Document Action column to open the Online Contract Collaboration view in classic. 

This configuration does not apply to Conga CLM (Contracts for Salesforce).

To configure the Document Action column

  1. Go to Setup > Build > Create > ObjectsDocument Version Detail.
  2. In the Document Version Detail, click the Edit action in the Page Layout section.
    This displays the Document Version Detail Layout page.
  3. Click the wrench icon () in the Related Lists section for Version Details.
    This displays the Related List Properties - Version Details popup.
  4. In the Available Fields column, select and add the Document Action column.
  5. Click Ok and click Save.
    The Document Action column is added to the Document Version Detail page.