OS Upgrade from 10.7 to 10.8.5 (Mountain Lion) – Problems in opening word, Excel and Power Point files after upgrade to ML

I upgraded my MAC from 10.7 OS to 10.8 (Mountain Lion). Now, I am not able to open any word, excel, powerpoint and pdf files that were on my hard drive. I do have Office for MAC 2011. I am able to open the same file from my email and save them to the hard drive and then re-open it. But I am not able to open any of the files that were already saved on my MAC hard drive prior to upgrading OS to Mountain Lion. Any suggestions or advise?