Either will work ofcourse, but the PDF-Lib is probably a lot cheaper than the word-convertion tools.
But with PDF you will have to make the documents by hand through PHP whereas a word doc utility would probably have some sort of one-size-fits-all converter that tries to make the best of your html (never doing exactly what you want)
Plus, PDF is much smaller than word and more portable.
But, if you take care of how long and wide your documents are, the html versions are quite printable...