Hi,

Please refer to the code below:

Python

from odoo import models, fields

class PurchaseOrder(models.Model):

    _inherit = 'purchase.order'

    attachment_ids = fields.Many2many(

        'ir.attachment',

        'purchase_order_attachment_rel',

        'purchase_id', 'attachment_id',

        string='Attachments'

    )

XML

<record id="purchase_order_form" model="ir.ui.view">

    <field name="name">purchase.order.view.form.inherit</field>

    <field name="model">purchase.order</field>

    <field name="inherit_id" ref="purchase.purchase_order_form"/>

    <field name="arch" type="xml">

       <xpath expr="//page[@name='products']" position="after">

           <page string="Attachments">

                <field name="attachment_ids" widget="many2many_binary"/>

            </page>

        </xpath>

    </field>

</record>

Hope it helps.