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/test_converter/tests/test_html.py
def test_image(self):
        field = self.get_field('binary')
        converter = self.registry('ir.qweb.field.image')
        with open(os.path.join(directory, 'test_vectors', 'image'), 'rb') as f:
            content = f.read()
        encoded_content = content.encode('base64')
        value = e(converter.value_to_html(
            self.cr, self.uid, encoded_content, field))
        self.assertEqual(
            value, '<img src="data:image/jpeg;base64,%s">' % (
                encoded_content
            ))
        with open(os.path.join(directory, 'test_vectors', 'pdf'), 'rb') as f:
            content = f.read()
        with self.assertRaises(ValueError):
            e(converter.value_to_html(
                self.cr, self.uid, 'binary', content.encode('base64'), field))
        with open(os.path.join(directory, 'test_vectors', 'pptx'), 'rb') as f:
            content = f.read()
        with self.assertRaises(ValueError):
            e(converter.value_to_html(
                self.cr, self.uid, 'binary', content.encode('base64'), field))
class TestSelectionExport(TestBasicExport):