There are several open source PDF generation libraries available for PHP. Here are some of the best ones:
TCPDF is a powerful PHP library for generating PDF documents that supports a wide range of features including graphics, colors, and page formats.
DOMPDF is a PHP library that converts HTML and CSS documents into PDF format. It supports advanced features such as font embedding, page headers and footers, and background images.
FPDF is a lightweight PHP library that allows developers to generate PDF documents on the fly using basic text and graphics.
MPDF is a PHP library that supports UTF-8, RTL, and HTML5, and can convert HTML pages to PDF documents.
PHPPdf is a PHP library that supports generating PDF documents from XML templates, HTML, or directly using API calls.
These libraries are all open source and have active communities of developers contributing to their development and maintenance. When choosing a PDF generation library, consider the specific features you need and the level of customization required for your project.