Help, FPDI is changing the size of my PDFs

I'm using FPDI to add some stuff to some existing PDF documents. Works great, except that it's slightly changing the size of the PDF document (the physical page size, not the file size), which is unacceptable since this is for a high-end print file. I've stripped out all the code to the bare bones to try and debug this. Here is what I have: $pdf = new fpdi(); $pdf->AddPage(); $pdf->setSourceFile('D:\\DocShare\\'.$filename); $tplidx = $pdf->importPage(1); $pdf->useTemplate($tplidx); $pdf->Output('newpdf.pdf', 'D'); When I have PHP output the raw original PDF file, it measures 8.34"x11.12". If I process it with the above code, the output measures 8.27"x11.7" with a blank white band added along the bottom. Any suggestions? I just need the new file to be the same size as the original. Thanks!