Instead, when you single-click on a PDF attachment in Microsoft Outlook, nothing happens and you can’t preview the PDF attachment in the Reading Pane. You may find yourself in a situation, however, in which that normal operation is no longer working. That’s because File Preview is typically on by default in Microsoft Outlook and all other Office applications.
Usually, if you receive an email message with a PDF attachment in Microsoft Outlook, you can simply click it once to preview it in the Reading Pane without downloading it. How to ensure you can preview PDFs in Microsoft Outlook with single click on a PDF attachment
