I often use the
pdftk utility to merge and split pdf files, but I always forget the argument order.
So, for the sake of posterity, here are the two main uses of pdftk:
Merging two (or more) pdf files:
pdftk fst.pdf snd.pdf cat output output.pdf
Extracting pages out of a pdf (range is 47 - 94 here):
pdftk input.pdf cat 47-94 output output.pdf