It's certainly feasible, and you have two options. Either learn the PDF format and create your pdf files from scratch up, or make use of existing PHP code written by someone that went through the process of creating a generic pdf writer.
Iirc, the PDF file format spec is downaloadable at adobe, but creating a generic writer from this spec will take quite a bit of time. As such, I'd recommend googling for "php pdf creation" or something similar. That's what I did a while back when I needed to create PDFs. It took me less than 10 minutes to find one that was free. Granted, it was written for PHP 4, needed some tweaking and wasn't always user friendly, but it did work... Still, you may be better off spending more time finding one written for PHP5, and preferably one that was completed/updated in the last couple of months as well.