You can set all fields to readony, using a context value and overriding the fields_view_get() method of your model

Example:

If you want to turn the entire account.invoice form to readonly, you can use the next code, using a context value:

Calling from any specific button, menu or field set a context value. I choose 'turn_view_readonly' value to True
from a the sale order view xml button.

<button name="action_view_invoice" string="View Invoice" type="object" class="oe_highlight"        
  context="{'turn_view_readonly':True}"/>

And  inherit  account.invoice code model, checking for this value

from odoo import models, api
from lxml import etree
import simplejson

class AccountInvoice(models.Model):
    _inherit = 'account.invoice'
    @api.model
    def fields_view_get(self, view_id=None, view_type=False, toolbar=False, submenu=False):
        context = self._context
        res = super(AccountInvoice, self).fields_view_get(view_id=view_id, view_type=view_type, toolbar=toolbar,
                                                           submenu=submenu)

        if context.get('turn_view_readonly'):  # Check for context value
            doc = etree.XML(res['arch'])
            if view_type == 'form':            # Applies only for form view
                for node in doc.xpath("//field"):   # All the view fields to readonly
                    node.set('readonly', '1')
                    node.set('modifiers', simplejson.dumps({"readonly": True}))

                res['arch'] = etree.tostring(doc)
        return res

You can also look for 'active_id' or 'active_ids' value in context, and check the state field of the recordset and turn the fields to readonly..

if context.get('active_id') and self.browse(context.get('active_id')).state == 'X':
      doc = etree.XML(res['arch'])   
      # ... etc