From the account.invoice it self

pdf = self.env.ref('account.account_invoices').sudo().render_qweb_pdf([self.id])[0]
self.env['ir.attachment'].create({
    'name': self.number + ".pdf",
    'type': 'binary',
    'res_id': self.id,
    'res_model': 'account.invoice',
    'datas': base64.b64encode(pdf),
    'mimetype': 'application/x-pdf',
    'datas_fname': self.number + ".pdf"
})

From other models

there should be a field in relation with account.invoice, here invoice_id

invoice_id = fields.Many2one('account.invoice')

pdf = self.env.ref('account.account_invoices').sudo().render_qweb_pdf([invoice_id.id])[0]
self.env['ir.attachment'].create({
    'name': invoice_id.number + ".pdf",
    'type': 'binary',
    'res_id': invoice_id.id,
    'res_model': 'account.invoice',
    'datas': base64.b64encode(pdf),
    'mimetype': 'application/x-pdf',
    'datas_fname': invoice_id.number + ".pdf"
})