Reduce the file size by copressing with ghostsctipt (to be installed):
˜ gs -sDEVICE=pdfwrite -dCompatibilityLevel=1.4 -dPDFSETTINGS=/prepress -dNOPAUSE -dQUIET -dBATCH -sOutputFile=compressed_PDF_file.pdf input_PDF_file.pdf
prepress (default) Higher quality output (300 dpi) but bigger size
ebook Medium quality output (150 dpi) with moderate output file size
screen Lower quality output (72 dpi) but smallest possible output file size
concatenatin pdfs
˜ pdftk first.pdf second.pdf cat output merged.pdf
pdftk is quite powerfull!