I'm making this post because I recently ran into this problem when making some last minute changes to a poster. When I went to save my .ai document as .pdf, Illustrator would hang/freeze and then crash. Some googling on the topic eventually led me to the solution.
The trouble lies in corrupt preferences settings. Not sure how this file got damaged, but the fix is simple. First, locate your Adobe Illustrator settings. For me (windows machine) I found it in my *hidden* AppData folder.
Make sure Illustrator is closed and locate the folder "Adobe Illustrator CS4 Settings" and delete it. I made a back up copy just in case something went horribly wrong, but deleting this should be fine.
Then hold down Ctrl+Alt+Shift when you open Illustrator. A dialog may pop-up saying something about the preferences. Now that Illustrator is up and running, open your document and try saving to .pdf. Problem should be solved!