I use addTTFfont() to convert existing truetype/opentype file into raw format, but it seems that tcpdf could not deal with file with large filesize, such as newly published Adobe Source Han Sans:
http://sourceforge.net/projects/source-han-sans.adobe/files/
The memory limits are imposed by your system and php settings.
Be also sure that the font is in one of the formats accepted by TCPDF.