How come Chrome opens some PDFs directly while DL others?
Solution 1:
The reason is pretty simple. The server actually makes the decision to either download, or render the PDF in the browser. So the server admin must set the server to do either option, or the web developer / webmaster can add code to force the PDF either way.
On your end, you can run a plugin called redirector, that changes the http-header that tells the browser to download the PDF, and actually make it do the opposite.
You create a rule that removes the Content-Disposition header.