How to insert unicode character in LaTeX document?

As far as I know there is no font for pdflatex that has U+20B4 HRYVNIA SIGN ₴ (the symbol for the Ukrainian currency).

You can emulate it, with something like

documentclass[a4paper]{article}
usepackage[ukrainian]{babel}

usepackage{graphicx}

DeclareRobustCommand{hryvnia}{{%
  fontencoding{OT1}upshape
  settoheight{dimen255}{S}%
  vphantom{S}%
  smash{ooalign{%
    hfilreflectbox{S}hfilcr % a reflected S
    hfilraisebox{0.25ex}{--}hfilcr % upper bar
    hfilraisebox{-0.05ex}{--}hfilcr % lower bar
  }}%
}}

DeclareUnicodeCharacter{20B4}{hryvnia}

begin{document}

Ви заборгували мені ₴2000.

end{document}

The definition of hryvnia maybe is not the best, but the idea is this: you find out how to print the symbol and then use DeclareUnicodeCharacter to get it when you type in the Unicode character.