Yes .. this is a long-standing and well known issue with Exchange and attachments sent to OSX Mail and other mail clients.  A quick google for winmail.dat or even search here at Experts Exchange for loads of references over the years

There are plenty of explanations but basically Outlook and  Exchange use a method of encoding attachments that not all other email clients can parse properly and OSX Mail is one of them

Solutions range from
1. Using Plain Text formatting in the email not HTML or RTF sometime can resolve it.
2. Change sending settings on Outlook for Windows on a per contact or overall settings basis
3. Changing the autocomplete email which may be defaulting to RTF even though you're sending HTML format
Remove the Autocomplete entries for the specific address(es) using NK2Edit.
Added email addresses to Contacts as new entries (Not Suggested Contacts).
4. Manage TNEF formatting at a server level depending on what mail server you're using
https://msdn.microsoft.com/en-us/library/gg263346(v=exchsrvcs.149).aspx

See lots of the above options in more details here
https://www.slipstick.com/problems/outlook-is-sending-winmail-dat-attachments/

Thanks Eoin

Bizzarely the same email sent to my iPhone arrives correctly, PDF included

Her Outlook is configured with an Imap ISP email account; no Exchange here

OK .. then I'd focus on the AutoComplete contacts on HER outlook .. item 3 in my posting.   Outlined in more detail in the URL I posted also

Right click AutoComplete contacts on HER outlook and select Properties and in Internet format field select "Let's Outlook decide" in the pull down menu.

The better way is to ask her to create a contact for every receipent instead of relying on the AutoComplete or Suggested contacts.

However, the above workaround is only feasible if you can manage the sender to change his or her settings in MS Outlook.

For most of the times, you can do nothing to ask the sender to change anything and the only option is to install an app like the one below.

Winmaildat Opener by Tobias Reinhardt
https://itunes.apple.com/us/app/winmaildat-opener/id596413390?mt=8

The user is back at her office and I was able to run another test

I modified the properties for her contact, by specifying that mail format should be in text mode

I then sent her an email with PDF attachment and this was again received as winmail.dat

The email was apparently received as text because she confirmed that her company logo wasn't present in the received email