I'm coding a program to make a pdffile including Japanese word.

In the following code,it writes "Hello" in document(txt.TextUnicode = "Hello")

PdfDocument doc = PdfDocument.CreateNew();
var font = PdfFont.CreateFont(doc, "YuMincho");//Yumincho is a popular Japanese font.
string text = "Hello";
var txt = PdfTextObject.Create(text, 300, 300, font, 20);

But When I change the 3rd line to

string text = "こんにちわ";　//Hello in Japanese

It writes nothing( txt.TextUnicode = "").

When I change the 3rd line to

string text = "こ";

It writes nothing but txt.TextUnicode is "ÿ".

I can't understand the reason, so please teach me　some solution.