As discussed in that thread, files that use "Content-Disposition: attachment" will not open in the builtin PDF viewer in Firefo and presumably Chrome.
I can confirm calibre delivers files using "Content-Disposition: attachment".
Code:
[eschwartz@arch ~]$ curl -I "http://127.0.0.1:8080/get/pdf/Clan%20Rathskeller%20-%20Kevin%20Hearne_726.pdf"
HTTP/1.1 200 OK
Content-Length: 151661
Content-Disposition: attachment; filename="Clan Rathskeller - Kevin Hearne_726.pdf"
Server: calibre/2.34.0.r24.g66b9085
Last-Modified: Thu, 06 Aug 2015 05:05:32 GMT
Date: Sun, 16 Aug 2015 02:55:10 GMT
Content-Type: application/pdf