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.
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.
FPDI transforms a pages content stream into a reusable structure called "form XObject".
Dynamic content like form fields or any other page annotations can't be attached to a form XObject because they are related to the coordinates of the origin page and may be related to other objects which are simply not supported by the PDF generation library FPDI is used with. So FPDI is not able to import this kind of content.
With FPDI 2.4.0 link annotations with URI actions will be imported if the
$importExternalLinks parameter of the importPage() method is set.
Also content that is related to the whole document will not be imported: Bookmark Outlines, Optional Content Groups (Layers), Actions,...