One of the my websites I have noticed that some end-users have been complaining that PDF wouldn't open on the initial try, they were using latest version of Firefox on Windows. We have a webpage that has PDF file, when clicked on a new tab is opened and PDF files would be opened within the browser. For some reason, when file is opened initially, file download would stop after first few seconds. Issue was experienced with files that were 500KB +. I'd say that it's end users' problem that PDF doesn't open, but it could also be a bug within the browser. I was able to achieve same error while trying to download some content from http://h10025.www1.hp.com/ewfrf/wc/manualCategory?cc=uk&lc=en&product=5199463 I had two PDF documents open at the time on my desktop, I have opened a manual from HP site, which didnt open, and PDF from my site, which didn't open either.