not sure if the approver needs to add the comment into the PDF document or not. If it can be outside the document, you maybe can leverage a SharePoint List and add a field with the comment (just as example) or add a dialog box that can be activated by adding a new field.
Another option is maybe use MS Teams as platform where you can send messages that can be forwarded to specific users.
To be honest, with so little detail is not so easy to provide more options.
Perhaps our friend @rezadorrani can provide more alternatives
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