Over the next couple of days Google is rolling out a new update to its Google Docs service that will enable you to provide access to users to view documents and allow comments, without full edit access.
Once its been added by Google to your Google Docs account, to give comment-only access to your document, click on the “Share” button. From there, add in the contact you’d like to share your document with, and select “Can comment”, as in the screenshot below.
Any users who have comment-only access to view your documents can add comments throughout the document, without the ability to change the content of the document directly.
You can also use the feature within Google Apps account. More information about the new Google Docs Comment Sharing feature can be found on the Google Blog.