The PDF is generated using the TCPDF library and we use special font called ‘rightpressnotosans’ font for characters. If you want to display Japanese characters, you will have to replace the ‘rightpressnotosans’ font with the one which supports Japanese characters.
1 - Download full TCPDF font pack from here:
2 - Extract and upload the fonts you have downloaded in ’\woocommerce-pdf-invoice\libraries\tcpdf\fonts’ folder.
3 - The font-family ‘cordiaupcbi’ works for most Asian languages.
You could also try those font families too: 'msungstdlight' and 'kozgopromedium'.
Replace the font name from 'rightpressnotosans' to one of these in both of these files:
4 - After this, generate a proforma invoice to check whether the characters are displayed properly.
PS: Note that this article applies to versions 3.1.8+. For previous you will need to replace "arial" font instead of "rightpressnotosans".