pdftk to the rescue (or: how to sign a PDF using a pen)

From the manpage: “If PDF is electronic paper, then pdftk is an electronic staple-remover, hole-punch, binder, secret-decoder-ring, and X-Ray-glasses. Pdftk is a simple tool for doing everyday things with PDF documents.”

True, but the usage isn't always intuitive. Well, maybe it would be if I did these things every day. Which I don't. This one thing, that I have to do every few months (trying to make it less frequent), goes something like this:

• Receive a PDF; call it orig.pdf
• print page 3, sign, and scan; call that sigs.png
• Create a new PDF which is the old one for pages 1–2, the scanned image from the previous step, and the old one for page 4; save as signed.pdf

Here is my cheat sheet for next time:

$ convert sigs.png sigs.pdf
$ pdftk A=orig.pdf B=sigs.pdf cat A1-2 B1 A4 output signed.pdf

There. Now the next time I have to do it, I won't have to stare at the manpage and try to reconstruct this incantation.