At it's simplist, you can attach a password to the file to protect it's contents from editing.
Here's how. Set MS_PROTECTION_ENABLE = 2, or to use a certificate, choose one of the other options.
Set to 0 to disable file protection.
Set to 1 to enable password or certificate protection.
Set to 2 to enable password protection only.
Set to 3 to enable certificate protection only.
Note, the following variables can also be set if desired:
MS_PROTECTION_PASSWORD_MIXED Set to 1 to require file protection passwords to be a mixture of letters and numbers. Set to 0 to allow passwords that contain only letters. The default is 0 (numbers not required). A password containing a mixture of letters and numbers is harder to guess than a password containing only letters.
MS_PROTECTION_PASSWORD_MINIMUM Sets the minimum length of a file protection password. The maximum is 80. The default is 5. A password of less than 5 characters is relatively easy to guess, thus making a protected file vulnerable to unauthorized access.
Once the variables are set, open the file in question and choose File > Protection > Protect
Enter a password and confirm.
In the Digital Rights dialog, choose Add Access for Everyone.
In the License Properties, key in a purpose such as "for viewing only".
Add an expiry if you want. And click OK.
Close the file and try to open it. If you supply the password, you'll be able to edit it's contents. However, If the user doesn't know the password, and just clicks OK, then the file will be opened in view only mode. In this case it can also be attached as a reference for viewing only.
File protection is a large topic and you can learn more by reviewing Help > File Protection.
Pick the tool. Adjust the settings. Follow the prompts.