Five years ago I noted that sometimes I get sent PDF contracts where I need to add a little bit of text, or maybe add my signature (which I have as a PNG), and I suggested a long-winded approach of converting to PS and editing in OpenOffice. Fortunately, this is a lot easier now: install Xournal (Dash > Applications > Xournal > install), and that has “add text” and “add image” tools. Add the text you want; add the image(s) you want; save as PDF. And Robert’s your mother’s brother. Much easier. Well done.
Update: actually, Libreoffice Draw, as recommended by a couple of people, is even better. It can happily open multi-page PDFs, it can edit the actual text in them rather than just overlaying, and Xournal did seem to create a PDF where my annotations weren’t readable on some devices, presumably owing to Font Issues. So, I rescind my above advice and change it to this: use Libreoffice Draw. Well done Libreoffice people.