FPDF is a PHP class which allows to generate PDF files with pure PHP, that is to say without using the PDFlib library. F from FPDF stands for Free: you may use it for any kind of usage and modify it to suit your needs.
FPDF has other advantages: high level functions. Here is a list of its main features:
FPDF requires no extension (except Zlib to enable compression and GD for GIF support). The latest version requires at least PHP 5.1.
The tutorials will give you a quick start. The complete online documentation is here and download area is there. It is strongly advised to read the FAQ which lists the most common questions and issues.
A script section is available and provides some useful extensions (such as bookmarks, rotations, tables, barcodes...).
The class can produce documents in many languages other than the Western European ones: Central European, Cyrillic, Greek, Baltic and Thai, provided you own TrueType or Type1 fonts with the desired character set. UTF-8 support is also available.
Of course, the generation speed of the document is less than with PDFlib. However, the performance penalty keeps very reasonable and suits in most cases, unless your documents are particularly complex or heavy.
For any remark, question or problem, you can leave a message on the forum (you don't need to register).
You can write to me here (but please use the forum for basic questions).