That setting is when Thunderbird retrieves and saves the document. When you open the document, it becomes under control of your PDF viewer software, which may save in a different location.

thx, but..

when I download a pdf from my firefox browser (or gmail in firefox) it goes to c:users/public/downloads.

if my pdf viewer software was set to a different location it would not go there from firefox either. it would also go to appdata/local/temp/pid like when I download from TB.

I think you're confusing 'download' with 'open.'. When Firefox downloads a PDF, it is not invoking the PDF viewer software. When you open a PDF, you are. And it's the PDF viewer that is downloading to the other location.