From a newsgroup post: "There is currently a bug in the Adobe .pdf IFilter that causes indexing to stale. We are currently working with Adobe to fix this issue. In the mean time, you can fix this issue by disabling all file extensions associated with the Adobe IFilter. Here are the steps you can follow:
1. Open your Control Panel and then Open Indexing Options.
2. Click the Advanced Button.
3. Inside the Advanced Options window, select the File Type tab.
4. Look for the .pdf extension and notice the Filter description.
5. Uncheck the .pdf extension and then look for any other extensions that have the same filter. Uncheck those extensions as well.
6. Restart your PC".
This event might occur when you are using WDS 3.0. It is a bug in Office 2007 beta 2 and will be fixed in the next release according to MSFT.