Add images or text to a pdf

A common situation is the need to add/overlay images, text, or whatever to an existing pdf called original.pdf, for example your signature. In Linux, this is quite simple by using a technique where you stamp one pdf onto another. I will use Ubuntu to demonstrate:

1. Create a pdf called stamp.pdf with the stuff you want to add, for example using LibreOffice Writer and exporting to pdf. Try to position your graphics approximately in the correct positions for overlaying the original pdf. You must have the same orientation (landscape/portrait) in both documents. The combined pdf will be called new.pdf.
2. Install pdftk:
   sudo apt-get install pdftk
3. Run the following command:
   pdftk original.pdf stamp stamp.pdf output new.pdf
4. Repeat from Step 1 with different positions of your signature, text, whatever until you are happy.