I should add that I tested this on several different documents -- .ai and .pdf files, files I created and files others created -- and it's still the same problem.
try and reset the Illustrator preferences...
Do one of the following:
- Press and hold Alt+Control+Shift (Windows) or Option+Command+Shift (Mac OS) as you start Illustrator. Your current settings are deleted.
- Remove or rename the AIPrefs file (Windows) or Adobe Illustrator Prefs file (Mac OS). New preferences files are created the next time you start Illustrator.
<startup drive>\ Documents and Settings\<username>\Application Data\Adobe\Adobe Illustrator CS5 Settings\en_US*\AIPrefs
Windows Vista or Windows 7
<startup drive>\Users\<username>\AppData\Roaming\Adobe\Adobe Illustrator CS5 Settings\en_US*\AIPrefs.
Note: In Windows XP, the Application Data folder is hidden by default. The same is true for the AppData folder in Windows Vista and Windows 7. To make this folder visible, open Folder Options in Control Panel and click the View tab. In the Advanced Settings pane, select the Show Hidden Files and Folders or Show hidden files, folders, or drives option.