Outlook sends .pdf files as .dat

Try using HTML as the email format instead of rich text.

To prevent Outlook from attaching winmail.dat when you send an email:

    Click File in Outlook.
    Select Options.
    Go to the Mail category.
    Make sure HTML or Plain Text is selected for Compose messages in this format: under Compose messages.
    Now make sure Convert to HTML format or Convert to Plain Text format is selected for When sending messages in Rich Text format to Internet recipients: under Message format.
    Click OK.

Hi DoctorK,
This is a very common, nasty problem, and there is no one solution that works for everyone. If you search EE by putting "winmail.dat" in the search box above, you will get 921 hits with thousands of views among them. I suggest reading some of those threads and trying some of the fixes that worked for other members, but this is definitely a YMMV (Your Mileage May Vary) issue.

I've participated in many questions here at EE on this same problem. Here are several of them:

https://www.experts-exchange.com/questions/28616021/People-receiving-PDF-attachement-as-winmail-dat-files.html

https://www.experts-exchange.com/questions/28129365/Pdf-file-recieved-as-winmail-dat-file-Outlook-2010.html

https://www.experts-exchange.com/questions/28199110/File-Type.html

https://www.experts-exchange.com/questions/28605102/pdf-files-converts-to-winmail-dat-file-when-sent.html

https://www.experts-exchange.com/questions/28512378/PDF-attachement-shown-as-WINMAIL-DAT.html

I hope one of these has a solution that works for you. Regards, Joe

Both answers very helpful.   Thank you.

> Both answers very helpful.   Thank you.

You're welcome. But if "both answers [were] very helpful", why did you mark just one of them as a Solution?