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.
return self.html_url_format % self.html_url_args
    def stat(self):
        if not (self.inline or self._filename or self._ir_attach):
            path = filter(None, self.url.split('/'))
            self._filename = get_resource_path(*path)
            if self._filename:
                return
            try:
                
                fields = ['__last_update', 'datas', 'mimetype']
                domain = [('type', '=', 'binary'), ('url', '=', self.url)]
                ira = self.registry['ir.attachment']
                attach = ira.search_read(self.cr, openerp.SUPERUSER_ID, domain, fields, context=self.context)
                self._ir_attach = attach[0]
            except Exception:
                raise AssetNotFound("Could not find %s" % self.name)
    def to_html(self):