The commenting mode in your documents is allowed only on drafts you own or have been shared with you.
If you have View permissions, you cannot edit the draft but you will always be able to add comments to it:
If a draft is shared with you, you are able to add comments to it with any type of permission. You can directly use the link to the shared drafts from the email notification to add comments to them.
Once a draft becomes a contract commenting mode will be disabled.
With Contractbook comments you can increase collaboration and transparency when working on your drafts. Learn how to add comments to drafts and mention external collaborators by tagging them in your comments here.