Edit ~/.config/fontconfig/fonts.conf

And between the <fontconfig> tags add:

1<!--nice looking fonts starts-->
2 <match target="font">
3  <edit name="autohint" mode="assign">
4   <bool>true</bool>
5  </edit>
6  <edit name="hinting" mode="assign">
7   <bool>true</bool>
8  </edit>
9  <edit name="hintstyle" mode="assign">
10   <const>hintslight</const>
11  </edit>
12 </match>
13<!--nice looking fonts ends-->

then rebuild the fonts cache:

1fc-cache -frv

Source: https://discuss.getsol.us/d/4156-okular-font-smoothing/7