In order to support Cyrillic characters you will need to load one of
your own fonts. DOMPDF does not currently support Unicode for the
built-in fonts. It is something we're looking into, but I don't know
that it'll be ready for the next release.
To load a font you'll need to have access to the ttf2ufm binary.
Alternately you can try the web-based font prep tool (currently in
alpha).