You can extract the pages from a pdf (or from several) and join them in another one:
pdftk A=one.pdf B=two.pdf cat A1-7 B1-5 output combined.pdf
You can extract pages from a pdf using Ghostscript:
gs -sDEVICE=pdfwrite -dNOPAUSE -dBATCH -dFirstPage=1 -dLastPage=7 -o extracted.pdf one.pdf