Do you need support for PDF documents with a version higher than 1.4? Check out the new FPDI PDF-Parser!
Yes, it's just a fake and not a real concatenation. As FPDI only imports single pages the structure of the resulting document is completely different from the one of the original.
As of FPDF version 1.6 it is possible to use different page dimensions in one document, which gives you the freedom to use several different pdf documents with different page sizes with FPDI! Before FPDF 1.6 it was only possible to use same sized page dimensions.
(For real concatenation see: SetaPDF-Merger API - this API works without modifying the internal structur of PDF docuements and has none of the stated limitations.)