How do I convert a winmail.dat file back to it's original pdf format?

When people send messages from incorrectly configured Microsoft Outlook email clients, a file attachment called winmail.dat may be added as an attachment to the message. This file contains formatting information for messages that use Microsoft's proprietary TNEF standard and any attachments sent with the original message. The file is not recognized by other email clients. You should reply to the sender with this Microsoft Support page: https://support.microsoft.com/en-us/topic/how-email-message-formats-affect-internet-email-messages-in-outlook-3b2c0536-c1c0-1d68-19f0-8cae13c26722

I haven't run across a winmail.dat file in a very long time. Whoever sent you the attachment must be using very old software for the email client.

I've used this free Mac App Store app in the past to extract data from the dreaded winmail.dat file(s):

https://apps.apple.com/us/app/winmail-dat-opener-dat-reader/id990497462?mt=12

What a great answer! Change the rest of the world!!! I use both systems and unfortunately this "winmail.dat" is a real problem.