odoo-survey-addons/survey_custom_matrix/views/survey_user_input.xml

<?xml version="1.0" encoding="utf-8"?>
<odoo>

  <record id="survey_user_input_form" model="ir.ui.view">
    <field name="name">survey_custom_matrix survey.user_input form</field>
    <field name="model">survey.user_input</field>
    <field name="inherit_id" ref="survey.survey_user_input_form"/>
    <field name="arch" type="xml">
      <xpath expr="//form//header//button[@name='action_view_answers']" position="after">
        <button name="%(action_view_pdf_answers)d" states="done" string="Print PDF Answers" type="action"  class="oe_highlight"/>
      </xpath>
    </field>
  </record>

</odoo>