Rich web site of the company which has invented PDF format and which still maintains it.
FPDF :
Web site which describes the original PHP package created in 2001 which gave rise to FPDF4ZOS tool.
TCPDF :
An offshoot of FPDF class which has countless additional features. But the TCPDF class is almost 20 times larger than the additional FPDF one.
PDF Toolkit or QPDF
Very useful freeware tools with which many operations may be performed on existing PDFs: merge page into a PDF, burst a PDF into single pages, add a watermark, compress-uncompress PDFs, protect-unprotect PDF, fill fields in a form, field flattening, dump PDF properties and bookmarks, etc.,