Since I was the one introducing Media viewers into this topic, sorry if this thread goes a bit off topic.
I tested the latest Media Viewers with Nightly downloaded today. I could not replicate any problems, either with pdf:s or office document transformed to pdf:s. Tested with Chrome, Firefox on Chrome, IE on win 7. Tested with small pdf:s and one 30Mb pdf with 1300 pages.
So it may be that one specific pdf that is the problem, does it work for others? We try to keep up to date with the pdf.js lib, but we are slightly behind.
If you do want to sort it out, start a new thread where you post alfresco.log, and check tomcat/logs/catalina.out, it may be some transfer error there. Be specific on what browser and os you are using.