Sage, basically what a toolkit like TCPDF does is embed additional content into an existing file. Its kind of like if you put the blank character sheet into a typewriter, manually aligned it, typed your text, moved to the next point, typed... etc... Now, imagine that your typewriter had access to a handful of different fonts, and you could set them to bold or italic. It's not necessarily the cleanest solution, but it works in a pinch, and generally works well for filling out forms.