The Xpedition software is not officially supported on Windows 8 or 8.1 just yet. We target releasing official support for Windows 8.1 in Q1/2015. Please do not see my comment as any officially commitment from Mentor which only comes from our supportnet release page.
Now saying this, I don't think you will have much problems with using 7.9.4 PCB Viewer if you choose to use it on Window 8. Some of the issues you might have is specific to the start menu and tiles supported in Windows 8 do not work well with 7.9.4, also depending on the design size you may see some issues with graphics performance. While testing Window 8 we did find some bugs specific to the use of this OS with our solution but I do not recall any show stoppers that would prevent you from using the PCB Viewer. If you do run into a problem using the PCB Viewer, since it's not officially supported, Mentor will not be able to provide you support unless this same issue can be reproduced in Windows 7.
Product Marketing Director