When printing PDF i don't see transparency for some objects?

Yes that's right, by default we give priority to speed of printing, the use of transparency object in a PDF is consuming a lot of resources (including memory and processor's time), which results in some black boxes or missing items.
 To change this, use the alternate method to print in the options: 

> Click on left menu 'Settings', then section 'Advanced', then set the option to use alternate print method.