Merge PDF Files

Join in1.pdf and in2.pdf into a new PDF, out1.pdf
  pdftk in1.pdf in2.pdf cat output out1.pdf
  or (using handles):
  pdftk A=in1.pdf B=in2.pdf cat A B output out1.pdf
  or (using wildcards):
  pdftk *.pdf cat output combined.pdf

• File → Create PDF → From Multiple Files…
• in Acrobat 6, there is a problem with the default order of merging documents and it almost always requires user intervention to put the pages in proper order before completing the merge