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@Krid.Ji

For you i finally able to solve the issue.  Open this file /1.7.7.0/classes/pdf/PDFGenerator.php  line no  32

const DEFAULT_FONT = 'freeserif'; 

set this font in place of  

const DEFAULT_FONT = 'helvetica';  

then  same file  line 

138 function  setFontForLang   comment first 5 line and put the last line and then check it will show like this image 

// if (array_key_exists($iso_lang, $this->font_by_lang)) {

// $this->font = $this->font_by_lang[$iso_lang];

// } else {

// $this->font = self::DEFAULT_FONT;

// }

$this->font = self::DEFAULT_FONT;

N.B: In my case i have to comment this 5 line, because when i print invoice from back end the code is missing. you can check from customer front end without comment this 5 line, if pdf print fine. only the first change that is fine. The code describe, the pdf add font depand on customer langauge. You can set the default font to  freeserif for thai

Thank you