You can't store bold characters in a text file or anywhere else.
But, you can print bold characters, and you can display bold characters.
As for printing, as it has been pointed out earlier, check the printer manual for any bold printing commands.
There are tons of ways of displaying bold characters on a screen. Any language that can manipulate rich text formats can do it . Of course, COBOL can do it; but why bother when there are tons (I like the sound of 'tons') of utilities (yes, such as HTML) and add-ons that can interact with COBOL in manipulating rich text formats.
If you have specific requirements, we'll show you how to involve COBOL.
Dimandja