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

To configure the Document Action column

  1. Go to Setup > Build > Create > Objects > Document 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. In the Related Lists section for Version Details, click the wrench icon ().
    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 in the Document Version Detail page.