If you're seeing the rectangles instead of characters in .pdf document, that means the font which you're using hasn't your characters. It can be solved by importing custom font which has a support for your language.
To import new font, connect to your server via sftp or ftp, navigate to
app/code/Magetrend/PdfTemplates/view/adminhtml/web/fonts/pdf/
and upload the font in this directory. Font will be loaded automatically in the editor. Then you will need to change a font for texts which are in rectangles.
Font has to be in .ttf format. Not every .ttf fonts are supported by Zend Pdf library. In case the custom font won't work, just try another one. :)