Are there any pre-conditions or required settings to print/merge documents in 4.7?
I had a similar issue on an installation with PHPOffice. 

PHPOffice is automatically installed via composer in CiviCRM, but apparently composer fails to check for needed PHP extensions. In my case php-zip was missing, and after installing that everything went fine.

I am not sure if composer should check the requirements, or if they aren't properly configured. Eli's comment helped a lot debugging this for me!