Hi Pablo,

Please try this way,

<record model="ir.ui.view" id="purchase_order_form_inherit">
    <field name="name">purchase.order.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="//field[@name='attachment_ids']" position="attributes">
            <attribute name="attrs">{'readonly': [('state', '=', 'purchase')]}</attribute>
        </xpath>
    </field>
</record>

you can change the state to required state in which you want to set the field readonly

Hope this helps,

Regards