I don't know about you, but I personally don't like that Mozilla Firefox loads PDFs in a new tab, as I find it slower for viewing their content.
Instead, I prefer to download the PDF directly , and there's a very simple way to do this, as always, by using about:config.
The procedure as I mentioned is very simple.
- We open a new tab and type about: config
- We lie a little and promise to be careful… 
- Then we type the acronym pdf
- We look for the option pdfjs.disabled which must be disabled and we double click to put it on true.
That's it. To revert to the default, repeat the procedure and double-click on pdfjs.disabled to restore everything to normal.