To password protect and/or encrypt a document using Microsoft PowerPoint for Mac:

1. Open the MS PowerPoint for Mac file you want to encrypt and password protect.

2. Select File > Passwords

3. In the Password to open section, check Encrypt this presentation and require a password to open.

4. In the New password field, type a password. Passwords are case-sensitive.

5. Verify (reenter) the same password.

6. Click Set Password.

7. Click OK. The file is now encrypted and requires this password to open it.

8. Don't forget to save this password in a secure password manager and securely communicate this to any users or recipients of this file.

9. To reverse this, repeat the steps without entering a password. Save the document and it will no longer require a password to open it. 

NOTE:  Do not send an email with the file and the password in the same email. Find some other manner to communicate the password to users other than email if you will be emailing the file.