I had a couple of people ask me about this one after upgrading to Windows 8.1 when using InformationSource. When try to click on a preview of a document when searching the Core Concepts area you get a message “The Adobe reader has not been detected on your machine or display pdf in browser option is unchecked.”
If you get this message make sure you have Adobe reader installed. Click the Edit\Preferences.
When the Preferences are displayed click General and down the bottom of the page you will have a button to select the default PDF Handler. Click this button and you will be prompted a couple of times to accept the change.
Once you do this close any browser windows and open again you should be able to view previews in InformationSource.
If you still can’t then you might want to review the help Adobe provide for making sure your browser can use PDF documents.