I'm mostly interested in annotating a PDF file with text at a predetermined position. GUIs and command line utilities are both Ok, but only free software solutions, please. However, I included image additions for completeness.
To be clear, the annotations must be part of the PDF file, otherwise it is not useful.
There are two similar questions on Ask Ubuntu, but they are both a couple of years old. These are How can I add text and images (for example, a signature) to a PDF? and How can I edit a picture into an existing PDF file?
I've tried Xournal, which does work. However, I think a little tutorial about how to do this would be good, so you want to add a small tutorial on how to use Xournal to accomplish these tasks, please add an answer.
I also tried updf, which didn't work for me, though this answer and this one for example says it can. I rebuilt the package (which is pure Python) on Debian Wheezy, using the sources from the updf PPA. It seems quite primitive and the "Save As" dialog did not even have a save button. If other people have had different experiences, please post.
For each answer, please provide a brief tutorial with screenshots if appropriate, as to how you accomplished this task.