I have some PDFs which, when viewed in readers under Linux (tried atril from MATE and evince from GNOME), the ligatures (e.g. fi, fl) are missing, as can be seen from the example (it should be specifications):

But when I view the same PDF in the built-in reader of Firefox (pdf.js) the ligatures are there; thus I know the PDF is not defective.
This looks like either this bug a missing font, but how to discover which one? This is the list of fonts in the PDF file:

I am using Debian unstable (jessie). What I have of fonts installed is
ii ttf-bitstream-vera 1.10-8 all The Bitstream Vera family of free TrueType fonts ii ttf-dejavu 2.34-1 all transitional dummy package ii ttf-dejavu-core 2.34-1 all transitional dummy package ii ttf-dejavu-extra 2.34-1 all transitional dummy package ii ttf-mscorefonts-installer 3.5 all Installer for Microsoft TrueType core fonts