PDF files can contain data in two formats - U3D and PRC. CAD Exchanger only supports importing PDF files with data stored as U3D.
CAD Exchanger supports importing PRC files as a separate format, but despite this, there is currently no support for importing PDF files with data stored as PRC.
At this time it is recommend to save a PDF file as PDF-U3D instead of PDF-PRC. In this case the file can be imported.
Or convert PDF-PRC file to an intermediate file of a format supported by CAD Exchanger and export intermediate file to needed format using CAD Exchanger.