The above issue occurs only in - webkit web browsers like chrome and safari - which provides support for ligatures - whereas browser like firefox does not.
A ligature is a combination of two or more letters joined as a single
glyph
Root cause
This issue with missing characters is due to ligature support provided by these modern browsers - let me explain how
1.The tool while converting - it converts characters to glyphs using poppler for rendering - now these browser when they come across characters like tt tf ti ff fi consider them to be ligature and searches for glyphs corresponding to tt and not t t
2.Since they do not have their corresponding glyphs - they just skip the characters and renders the rest - so, we fount the characters missing
Could be solved by
Disabling/ Turning-off the ligature in these browsers - embedding the css in the generating content
For more details please refer:
Please correct me if I am wrong.