Beginning with Microsoft Outlook 2000 post-SP1 security patch, Outlook's reading pane is secure and reading messages in the preview or reading pane is actually slightly more secure than opening messages. Choose to use (or not use) the reading pane based on your habits and needs, not because you think it makes your computer more secure.
In addition to reading your email, you can also read attachments in the reading pane; we've included attachment preview handlers on this page.
If PDF files won't preview in Windows 64-bit, see Can't open or preview .PDF attachment in Outlook for the solution. Note that, at this time, .MSG files saved to the file system cannot be previewed in File Explorer if you have 64-bit Outlook installed; it works only with the 32-bit version.
Adobe Reader 8.1 and above installs a preview handler for Outlook and Windows or you can use FoxIt.
For those who use Foxit Reader, not Adobe, for PDFs, there is a PDF preview handler available for Outlook 2007 and Vista.
Adds preview handlers for .config, .psq, .xps, "xmlfile", .resources, CSV files, SNP files, MPE files.
If you want to preview additional file types in the Reading pane, you'll need to install preview handlers. Instructions for writing your own handlers along with some common file types are available in Stephen Toub's Vista and Office: View Data Your Way With Our Managed Preview Handler Framework article on writing managed preview handlers in the January 2007 issue of MSDN Magazine. (CHM format, from the MSDN Magazine archives)
Stephen Toub's preview handler editor utility: Preview Handler Association Editor
If you have an ActiveX control for a file type, see Get Preview of More File Types/Extensions in WDS 3.01/4.0 Preview Pane on Windows Xp for instructions to set it up as a preview handler.