Which program are you using to view your text files?

Look at the documentation for that program and see what you need to insert into the text in order for it to display in bold.

See the examples given previously..

If you are viewing the file in one of the Coifs (Chrome, Opera, IE, Firefox, Safari browser) then you will need to insert..

There is a <b>bold</b> word in this sentence.

Usually, the extension of the file name, eg .HTM, .RTF, .DOCX determines the format of the text inside, so (unfortunately) there is not one fits all.

etc