Problem:
okular doesn't show fi ligatures in PDFs with no embedded fonts.
Causes:
1) poppler/fontconfig replaces generic non-embedded fonts (e.g.
Times) with
tex-gyre fonts (e.g. /usr/share/texmf/fonts/opentype/public/tex-gyre
/texgyreheroscn-bold.otf)
2) Package fonts-texgyre sets high-priority substitution for Times
and related
fonts in /etc/fonts/conf.avail/65-fonts-texgyre.conf
Solution:
apt-get remove --purge fonts-texgyre
Upstream bug report:
https://bugs.freedesktop.org/show_bug.cgi?id=73291