When a PDF invoice is generated and emailed, the attached invoice will not open. However, when you view the booking online, the downloaded invoice opens with no issues. The email is being sent through Mandrill using CMandrill (not sure if that may be part of the problem or not). If you want to test, see http://www.msrecycles.org/fall2014confreg.html. I am using the latest version of Matukio (4.5.4).