pdftk

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