require an employee to sign a logbook without being able to change the content

We would like to use logbooks to provide a template form for performance reviews that managers and employees can sign after completion. Currently if we allow an employee access to sign, they can also make changes to the content of the logbook that has been completed by the manager. Is there a way of locking the manager comments so that they cannot be changed? or adding the logbook to documents so that it can be signed by the employee.