I would suggest that you generate just the first PDF without any Append and examine it. I suspect that it will have that blank page as the last page of the resulting PDF.
FileMaker will generate that blank page is there is just enough white space at the bottom of the body or trailing grand summary to keep it from fitting all on one page and then you get a trailing blank page. This can happen with just one pixel more of space than will fit in the printer's "printable area" given the printer driver, the page size, the page orientation and any margins specified in Layout Setup.
I am getting the same problem, and I thought it was likely a form issue too, rather than the append. I did a single pdf report, and shrunk the portal to less than a page, and it is still giving me the extra page. I'm now looking at PDF margin settings. Not sure what is causing the extra blank page in pdf's.