In order to combine several PDF files (eg scanned pages) into one PDF, the following works
gs -dBATCH -dNOPAUSE -sDEVICE=pdfwrite -sOutputFile=output.pdf input1.pdf ... inputN.pdf
and the argument
-dPDFSETTINGS=/ebook
can be used to reduce the quality. In order from least good to best
- screen
- ebook
- printer
- prepress
On second thought, it is probably better to use pdftk.
Which is perhaps a simpler solution.