PDF is not like HTML where you can apply formatting tags for emphasis. As you've read in the PDF reference, all that you do in PDF is to setup a graphics environment (colours used, fonts used, etc) and then put text on the page.
If you want to have something show in bold, use a font that is bold. If you want to have something show in italic, use a font that is italic.
Older software used dirty tricks to create "bold-alike" text, but the good (and easy) way to do it is to make sure you select the correct font before you start drawing text.
与恶龙缠斗过久,自身亦成为恶龙;凝视深渊过久,深渊将回以凝视…