Yes. It is necessary to customize the message inside the PO approval workflow.
It is also possible to for users to modify the subject / content of the email. You need to reference the subject and content to a workflow item attributes.
Will you please give example how we can referenced and change the subject.
In the Workflow Builder, expand the Messages > Email PO with PDF to the Supplier.
You will find the message attribute EMAIL_TEXT_WITH_PDF, and it is referencing the item attribute EMAIL_TEXT_WITH_PDF
You can programmatically update the value of the item attribute in order to modify the email content.