Web2PDF does exactly that, inserting a few lines of code will generate a “Save page as PDF” button on your pages. When the user clicks it, it will generate a PDF version of the page on the fly and allow him/her to download it.
Since it generates the PDFs on the fly, it will save you time, disk space and bandwidth. Also you don’t have to update your PDFs every time you update the articles. http://web2.pdfonline.com
I never could understand why the pdf reader needs so many updates i supposed it should be a a quite simple software