The Zend Developer Zone has posted a new tutorial today (from Richard Bates) about creating PDF forms from inside a Flex application with a little help from PHP.
Leveraging the power of PHP and a PDF generation library called dompdf, you can create a simple, seamless user experience in Adobe Flex. Flex enables you to create an outstanding presentation to the end-user, with instant validation of user-supplied data and the full power of ActionScript 3. Through AMFPHP, you'll gain access to PHP's full toolset, enabling virtually limitless applications.
They show how to install the needed packages - AMFPHP and the dompdf PHP package to make the PDF generation easy ("one of the best PDF tools for PHP you can get"). The rest is the creation of the Flex part of the application - generating the form and using PHP to push out a PDF file at the other end, complete with their data.