You are reading the documentation of the legacy and unsupported version 1 of FPDI.
For version 2 please refer to this documentation.
Limitations
Table of Contents
Compressed Cross-reference and Object Streams
Generally FPDI does not depend on a specific PDF version. Anyhow a general structure change was introduced in PDF 1.5 called "compressed cross reference streams" and "object streams". Reading PDF files that make use of these special structures are not supported by the open-source version of FPDI. We offer a separate parser addon that allows FPDI to handle these documents. More information are available here.
Encrypted Documents
Importing of pages from encrypted/protected documents is not supported.
Because FPDI will recreate a document from scratch it is useless to use an encrypted/protected PDF document as a source file.
Dynamic Content
FPDI transforms a pages content stream into a reusable strucutre called "form XObject". Dynamic content like link annotations, form fields or any other page annotations cannot be attached to a form XObject because they are related to the coordinates of the origin page. So FPDI is not able to import these content.
Also content that is related to the whole document will not be imported: Bookmark Outlines, Optional Content Groups (Layers), Actions,...