The GNU PDF project is developing a free, high-quality and fully functional set of libraries and programs that implement the PDF file format and associated technologies to the coming ISO standard, ISO 32000.
Why? Isn't PDF support "fine"?
According to José Marchesi, quoted by Bruce Byfield missing or incomplete features in Free Software PDF implementations include:
Contributions page for GNU PDF: https://www.fsf.org/donate/directed-donations/gnupdf.html