Because it is a Microsoft proprietary file type sent from a Microsoft software user that does not know how to configure their software to be compatible with the rest of the world. https://support.mozilla.org/en-US/kb/what-winmaildat-attachment

I understand the answer but doesn't tell me why I can open them in gmail as pdf but not in thunderbird. I can open the same email in gmail and it ccomes as a PDF but in thunder bird is't a winmail.dat file WHY

If it says it is a winmail.dat file it is not a PDF. There is page after page describing this problem so you can do the research. Even Microsoft has instructions for their users to set their system to stop sending these unusable file types.

I repeat I know the difference between a winmail and PDF My question is WHY it shows up in Thunder Bird as a WINMAIL and in gmail a PDF

I think we are done. I guess I cannot help you.

Because you don't understand the question

No I understand just fine. The info is well documented if you search so no need to type it out just for you. Thunderbird will not open winmail.dat files. There is an add on that claims to but I have never used it.

You still don't understand the question. If I open an email in thunderbird that has an attachment it appears as winmail. If I open the same email in gmail the attachment appears as a PDF.

WHY simple question

Gmail and TB have different methods of reading mail. If you install the LookOut add-on (how to install), you should be able to decode winmail.dat files and recover the underlying pdf files.

Again,I understand the situation. You seem unable to comprehend the answer. This is the support forum for Thunderbird. It does not work in Thunderbird. Ask in a gmail forum why it works there. Simple answer.