In jasper-reports section there is this jasper-report-character-encoding-in-pdf question.
The problem can not be solved in jasper report since it seems to be an itex
Please try the below codes,
string ZawgyiOnePath = Path.Combine(Environment.GetFolderPath(Environment.SpecialFolder.Fonts), "ZawgyiOne.ttf");
BaseFont ZawgyiOneBaseFont = BaseFont.CreateFont(ZawgyiOnePath, BaseFont.IDENTITY_H, BaseFont.NOT_EMBEDDED);
iTextSharp.text.Font ZawgyiOneFont = new iTextSharp.text.Font(ZawgyiOneBaseFont, 9);
Converting Bruno Lowagie (Original developer of itext) comment into answer (community wiki)
The behavior you see is caused by the fact that iText doesn't support ligatures. (Current version on github.com 5.5.8)
You need one of the next, unreleased versions of iText. We'll release beta versions next year (2016) but only to customers
Thanks to some bounty offered by @Rad Lexus this other question about accessing directly openType glyph in iText has an answer Accessing OpenType glyph variants in iText