I am trying to combine approx. 6 PDFs into a single PDF. The comnibed PDF wil have approx. 1400 pages when I am done. When I tried to combine the first 6 PDFs, I got a message that stated that the file could not be saved because it is too large for the current operation. The PDF would have had 990 pages in it and if I am successful into combining these 6 PFDs, I intend to add an additional 500 pages to the file. Why is Aerobat 9 Pro Extended not allowing me to combine the files?