On PHPMaster.com there's a new tutorial from Shameer C showing you how to use Dompdf to convert HTML to PDFs. The handy library makes it (relatively) simple to automatically generate the documents without the need to know the internal PDF file structure.
PDF is a standard format originally created by Adobe for representing text and images in a fixed-layout document. It’s not uncommon for a web application to support downloading data, such as invoices or reports, in PDF format, so in this article we’ll go through how we can easily generate PDF documents using PHP.
He starts off with the installation (via Composer) and the creation of a simple "Hello World" document, showing how to "stream" it into a PDF file. He also includes some more advanced usage - loading an external HTML, drawing lines, changing fonts, adding page numbers and additional text to the page.