Open up Adobe Reader/Adobe Acrobat and change the preferences to not use Adobe to open PDFs in Safari, then Safari can use its own renderer.
Of just deselect that option, which will remove the plugin for M$ Office and then it should work.
TEG
Close Safari, open a terminal and type:
defaults write com.apple.Safari WebKitOmitPDFSupport -bool false.
Open Safari and try a pdf.
What kind of a pdf file are you trying to open? What is the source?