The reason for the demonstrated behavior is that the PDF file is missing the required font for the special characters. The issue is not present when the characters are displayed in the browser, as the browser has the required font for the specific characters by default. The PDF file also has some default fonts that it supports, but these are different from the ones in the browser. Thus, if the PDF file has to render some characters that it doesn't recognize, then the reported behavior will occur. What should be done to avoid such behavior is to manually embed the required font in the PDF, as demonstrated in the following article: