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 $ gs -dNOPAUSE -sDEVICE=pdfwrite -sOUTPUTFILE=output.pdf -dBATCH file1.pdf file2.pdf file3.pdf

Jan. 26, 2012, 8:51 a.m.Janos

Explanation

Free PDF editing software might become more and more available, but this method has been working for a long time, and likely will continue to do so.

Limitations

It may not work with all PDFs, for example files that don't conform to Adobe's published PDF specification.