PCs running Outlook Express version 6 may receive this error message while attempting to open an Adobe Acrobat file attached to an email message.  This feature in Outlook Express is designed to prevent potential malicious files from infecting the PC. Unfortunately, this restriction may also prevent you from opening files that are safe but share the same filename extension (i.e., PDF Adobe Acrobat files). To open these files, perform the following steps:

  1. Start Windows Explorer (right-click on the Start button and choose Explore)
  2. From the Tools menu, select Folder Options
  3. Click the File Types tab
  4. Scroll to the extension PDF: Adobe Acrobat Document and select it
  5. Click the "Advanced" button to display the "Edit File Type" window
  6. Remove the checkmark from the Confirm open after download option
  7. Click OK twice, then close Explorer

This should immediately allow you to open attachments with that extension.