By default FPDI can "only" handle PDF documents up to PDF version 1.4. Beginning with PDF version 1.5 there were new compression features introduced which involve internal structure changes how a PDF document can be created.
With this seperate parser, as a commercial addon, you're up to date and FPDI will be able to handle PDF documents with a version higher than 1.4 without problems. Error messages like "Unable to find xref table." are history with this addon!