You can set more advanced protection options using the Protect Document task pane. To enable this task pane, ensure the Protect Document icon on the forms toolbar isn’t toggled. Then, click the Tools menu and click Protect Document. You’ll see this task pane appear on the right hand side:

For forms, you will want to check the second option and choose Filling in forms from the drop-down menu. Once you click the Yes, Start Enforcing Protection button in Step 3, this dialog will appear:

This allows you to set a password that will be required to unprotect the document. (You can leave the password fields blank for a password not to be required.) Click OK to apply the protection.

Now, when you click the Tools menu and click Unprotect Document, or click the Unprotect Form icon on the Forms toolbar, you will be prompted for a password.

If the password is entered correctly, the form will be unprotected so that it can be edited. If the correct password is not entered or the user clicks Cancel, they will be returned to the protected form.