How can I add text and images (for example, a signature) to a PDF?

There is no perfect way to do this yet. The best existing way is to install Xournal from the software center and open PDF files with it. It allows you to annotate them and then export the whole thing as a new PDF.

Since you are basically marking up a new layer on top of the original PDF, you have to line up everything as you type, and manually position all your notes. For a simple document it's pretty easy to do. It's not very practical for longer documents, though.

Open the pdf in LibreOffice Draw and copy/paste there any image, including that of a signature (or Insert -Picture - From File), which then can be easily adjusted/resized to fit the purpose. After that, under File - 'Export as PDF'