A new post on the Tor Blog says that they are going to add the PDF.js extension to TBB next month, allowing users to read PDFs in the browser. https://blog.torproject.org/blog/new-firefox-17-and-tor-alpha-bundles Apparently they are confident that they can make this safe and prevent links from going over clearnet since the extension is contained within TorBrowser. That should make the building of disposable VMs for reading PDFs obsolete.