That depends on the browser
Why does the upload addon think a .pdf file is a mime type application/download?
, because the browser passes this data to a server.
So far I have tested and can confirm, that Mozilla is considering PDFs as application/binary
Internet Explorer, Opera, Safari for Windows, Google Chrome - as application/pdf
Conclusion is that obviously you're trying to test this with Mozilla, which has huge amount of bugs (as always).
However the way the files are shown in the forum is a little sloppy...
It may appear that way because of some wrong modification you did. In a default version there should be displayed attachment_icons/pdf.gif
icon near the uploaded file. You may check how it looks on miniBB demo
(I already provided some test PDF example which 100% worked on my end).