Can't open email attachments sent from Windows Why does a PDF attachment on an email from a PC end up as a file named winmail.dat when it arrives on my MacBook Pro?
The sender is using the proprietary email client Microsoft Outlook.  The entire message is sent in a proprietary format to preserve Rich Text Formatting, according to Microsoft.

A Google search for Winmail.dat will reveal several work arounds to deal with these kinds of files.  The simplest, but most annoying, is to ask the sender to resend the email in plain text format.  This will prevent the attachments from being bundled into the winmail.dat file.

From Microsoft:


  The data in a winmail.dat file is not usable. To resolve this issue, ask the sender to re-send the message in plain text format.