I assume this is caused by something in that specific PDF. If it is a publicly available document, you could share the URL here or, even better, report the bug over on the Github Issues list at https://github.com/mozilla/pdf.js/issues If it's confidential, I'm not sure where to look for leads. Firefox's Browser Console (Ctrl+Shift+J) collects error messages from centralized functions like printing, so maybe you'll see something there at the point of failure?