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.
""" returns a :class:`~openerp.models.Model` used to render a
        ``t-field``.
        By default, tries to get the model named
        :samp:`ir.qweb.field.{field_type}`, falling back on ``ir.qweb.field``.
        :param str field_type: type or widget of field to render
        """
        return self.pool.get('ir.qweb.field.' + field_type, self.pool['ir.qweb.field'])
    def get_widget_for(self, widget):
        """ returns a :class:`~openerp.models.Model` used to render a
        ``t-esc``
        :param str widget: name of the widget to use, or ``None``
        """
        widget_model = ('ir.qweb.widget.' + widget) if widget else 'ir.qweb.widget'
        return self.pool.get(widget_model) or self.pool['ir.qweb.widget']
    def get_attr_bool(self, attr, default=False):
        if attr:
            attr = attr.lower()
            if attr in ('false', '0'):
                return False
            elif attr in ('true', '1'):
                return True
        return default
class FieldConverter(osv.AbstractModel):