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
uid = openerp.SUPERUSER_ID
        preprocessed = True
        for atype in (SassStylesheetAsset, LessStylesheetAsset):
            outdated = False
            assets = dict((asset.html_url, asset) for asset in self.stylesheets if isinstance(asset, atype))
            if assets:
                assets_domain = [('url', 'in', assets.keys())]
                ira_ids = self.registry['ir.attachment'].search(self.cr, uid, assets_domain)
                ira_records = self.registry['ir.attachment'].browse(self.cr, uid, ira_ids)
                for ira_record in ira_records:
                    asset = assets[ira_record.url]
                    if asset.last_modified > Datetime.from_string(ira_record['__last_update']):
                        outdated = True
                        break
                    if asset._content is None:
                        asset._content = ira_record.datas and ira_record.datas.decode('base64').decode('utf8') or ''
                        if not asset._content and ira_record.file_size > 0:
                            asset._content = None 
                if any(asset._content is None for asset in assets.itervalues()):
                    outdated = True
                if outdated:
                    if ira_ids:
                        self.registry['ir.attachment'].unlink(self.cr, uid, ira_ids)
                    preprocessed = False
        return preprocessed
    def preprocess_css(self, debug=False):