Thanks Sudhir

I tried same method,and also override the create method for restrict adding attachment is done state.it will works fine in local server.but i push in my production it will cause error when printing any pdf reports.
my code is like below,

class IrAttachment(models.Model):
# _inherit = 'ir.attachment'
#
# @api.model
# def create(self, vals):
# if vals.get('res_model') == 'purchase.order':
# po = self.env['purchase.order'].browse(vals.get('res_id'))
# if po.state == 'done':
# raise UserError('Can not add Attachments in Locked State')
# else:
# return super(IrAttachment, self).create(vals)
#
# @api.multi
# def unlink(self):
# if self.res_model == 'purchase.order':
# po = self.env['purchase.order'].browse(self.res_id)
# if po.state == 'done':
# raise UserError('Can not Delete Attachments in Locked State')
# else:
# return super(IrAttachment, self).unlink()