>Some possibilities:
>
>1. As of version 19 FireFox has a built-in PDF reader, so that or later versions should open the PDF in the browser. Earlier versions won't do that by default
>
>2. On the "download" machine, the browser not having any plugin (e.g. Acrobat Reader) registered to handle PDF content in FireFox
>
>3. Antivirus/antimalware software preventing direct opening of potentially malicious PDFs

Thanks, but after verification, from the same PC and browser, it works on one site and not on the other one. So, there seems to be some malformed HTML or something like that based on a condition to the specific request. We are simulating that on a test environment and I will be able to provide a resolution and will you know what was the cause.