This also happened to me and I was going crazy because the items might of been deleted or from a DVD that is no longer mounted. So this is how I solved it.
To summarize, you need to delete or rename some files in your local "Library" directory, ie ~/Library/Preferences directory.
For the iWork apps and Preview. Rename these following files with a "~" in the beginning or delete it or to be safe, copy to another location:
"com.apple.iWork.Keynote.LSSharedFileList.plist" to "~com.apple.iWork.Keynote.LSSharedFileList.plist"
do the same for these files
com.apple.iWork.Numbers.LSSharedFileList.plist
com.apple.iWork.Pages.LSSharedFileList.plist
com.apple.Preview.LSSharedFileList.plist
One way to do it through the GUI is open finder, go to the "Go" menu and press the "option" button.
Choose the Library menu item and open the "Preferences" directory.
Look for the files mentioned above either rename, delete, or move.
Close the application and see if the recent items still appear when you right mouse click the icon in the Dock.
If that doesn't work, then logout and log back in.
Hope this helps or solves your issue.