To add documents to the object you need to add the Additional Documents custom lookup to the Object.
The following Salesforce Objects: Account, Case, Contact, Lead, Opportunity include the Custom Lookup by default.
For other objects, you need to add Lookup from Additional Documents to the Object.
To add Lookup from Additional Documents to an Object’s layout:
Note that in Lightning, adding any Related List to the page layout does not automatically display it in the Lightning page.
Verify that you have added the Related Tab to your Lightning page layout, and add the Related Lists component to this Related Tab.
Only after this, are all related lists you added to the page layout for the record in the Object manager automatically displayed.
We will add the lookup to the Order object.
- Go to the Additional Documents object: Custom Fields &Relations (these lookups exist by default for OOTB objects).
- Click New.
- Select Custom Relations.
- Select the object to which you want to add the lookup (in our example: Order).
- Click Next.
- Click Next: The lookup to the new object (Order) now appears in the list.