Safely handling PDFs and other ebook formats on Linux

I'm running Arch Linux and using Okular for opening PDF files and FBReader for other ebook formats (Epub, Mobi, etc.). Simply put, here's my question: Assuming some of those documents come from unreliable sources and contain malicious code what can I do to mitigate the risk of compromising the system and opening it for invasion (which can be a common occurrence in this country if you even smell like someone who holds opinions the government disapprove of)? A few more specific questions: Is just opening the referred files enough to put my setup at serious risk? The user I use for this is on the sudoers list, so, if compromised, it could be used for escalation. Suppose I only open the files using a more limited user account, would that at least help? Outside of setting up a virtual machine only for reading (which wouldn't be practical for a few reasons) or using another computer just for that (same thing), is there anything I can do?