The Drawing library supports the output of Unicode characters only if the fonts you provide contain glyphs for the referenced characters.
Otherwise, depending on the individual font specifics, fonts render default glyphs which are typically blank rectangles.
To provide custom fonts, follow the steps in the article on embedded fonts.
Normally, the browser substitutes the missing characters (glyphs) from a fallback font. The PDF Export module has no access to the information about these substitutions. As a result, the substituted characters are rendered as the default glyph which is typically a rectangle. To work around this limitation, use fonts that contain all supported characters.