You are reading the documentation of the legacy and unsupported version 1 of FPDI.
For version 2 please refer to this documentation.

FPDI 1.x Introduction and Index

FPDI is an extension for FPDF that allows you to import single pages of existing PDF documents into a structure that can be reused with FPDF.

FPDI is published under the Apache Software License, Version 2.0.

The »F« in FPDI stands for »free«: you may use it for any kind of project and adapt it to your needs - the license conditions of ASL 2.0 are met nevertheless.