What is font substitution?
All postscript printers come with a standard set of built-in fonts. That is part of their ability to produce documents that look similar regardless of the printer technology and the printer’s manufacturer. PDF viewers have the same postscript standard fonts built in so that they can accurately display postscript printer fonts. Some fonts on personal computers are similar to some built-in postscript fonts but have different names and may differ slightly in appearance. When a postscript printer driver sees a font that is non-standard in a document it can look at some of that font’s characteristics and substitute the closest built-in font. Many times this substitution is quite accurate. An example is that the built-in Helvetica font often substitutes for the True Type Arial font.
If you open a document in a font that you don’t have installed on your system, Word displays it in a different font (usually Times New Roman) so that you can read it. Unfortunately, this means that Bangla is unreadable. Ekushey knows which fonts are Bengali and substitutes them with an appropriate alternative, so they are readable.