A Contact attempting to fulfil a Document Request may find that they are unable to attach a file or update the respondent comments:

• the Add Attachment button is missing from the Web application
• the Respondent Comments field is read-only

This is a permissions issue.  Often a Contact will be granted the Audit Viewer role (or similar custom role), and after upgrading to v6 the role has the Execute Document Requests permission set to None.  In order to grant access to this functionality, update the user's role such that Execute Document Requests is set to Own or All: