You must download the last version of jsPDF, there will be a file called jspdf.plugin.standard_fonts_metrics.js, just reference that script from your html page and that would be more than enough to user special caracters would be a simple line like this:
<script src="jspdf.plugin.standard_fonts_metrics.js"></script>
<script>
doc.text(74, 25, 'Evaluación de prueba');
</script>
This worked for me.