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.
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.
It was likely sent from a Windows PC using some variety of MS Exchange.
There is a fix but it has to take place on the Exchange server. And while it is a simple configuration change most Exchange admins don't see the need unless prodded from above.
You may have to get one of the utilities that decode a winmail.dat files. I thought I saw one the other day in the App Store. A quick search should turn it up...
I found that there were a few "freemium" programs promising to solve this, but the desired functionality - the ability to open and save the compressed attachments - was only available as an "in-app" purchase of the "premium" version for $10. I had no choice, as I needed the files to finish some time sensitive work from my home using my iMac. Microsoft Exchange has it's good points, but this is not a feature to recommend it. FYI, the app I purchased was called Winmail.dat Viewer - Letter Opener 9, Winmail.dat Premium.