Impacted versions: ODOO 10

Problem.
If i print a invoice with a negative amount, the pdf shows a vertical bar instead of a minus symbol.
![image](https://cloud.githubusercontent.com/assets/28800797/26237253/b07fadb0-3c74-11e7-86c2-c8688d82b5a5.png)

I have tried to install a new pdf reader, but with no result.
openerp/addons/base/ir/ir_qweb.py
raise NotImplementedError()
    @lazy_property
    def last_modified(self):
        try:
            self.stat()
            if self._filename:
                return datetime.datetime.fromtimestamp(os.path.getmtime(self._filename))
            elif self._ir_attach:
                server_format = openerp.tools.misc.DEFAULT_SERVER_DATETIME_FORMAT
                last_update = self._ir_attach['__last_update']
                try:
                    return datetime.datetime.strptime(last_update, server_format + '.%f')
                except ValueError:
                    return datetime.datetime.strptime(last_update, server_format)
        except Exception:
            pass
        return datetime.datetime(1970, 1, 1)
    @property
    def content(self):