I completed CS50 by 31st Dec. All my assignment were graded 1. But I still haven't got the link to download the PDF file for my certificate.
The 2014 offering of the course has the following text displayed in bright yellow on the front page.
If you did finish CS50x 2014 and met the syllabus's expectations for a certificate, your certificate should appear at courses.edx.org/dashboard as a downloadable, printable PDF by 31 January 2015. Don't worry if your dashboard says your final grade is 0%. CS50 doesn't use edX's built-in gradebook, so everyone's dashboard says that!
It isn't 31 January yet. So don't worry, there's still plenty of time.