As Bindu Tadivaka said sign up in that site. you will find different exams Test ID's there you have to select the exam ID 1Z0-851 . If you have netbanking or credit card there is no need to purchase vouchers directly you can book a slot there.Once you book a slot you will get confirmation mail and payment details. Take a print of that while appearing the exam.
I’ve looked at a lot of different solutions, and in my humble opinion Aspose is the way to go. Here’s the link: http://aspose.com