tcpdf either uses FreeType to rasterize fonts, or simply embeds them in PDF, and they are rasterized in the viewer. Ligatures are actually defined in fonts. If you use a Type1 font and specify a ligature character in your line, Freetype will rasterize it. Automatic substitution of "fi", "ff", etc. For ligatures, as in OpenType, FreeType is not supported, so I assume that it will not be supported by tcpdf either. See http://www.adobe.com/devnet/opentype/archives/glyph.html for glyph names, etc.