PDFEdit is the PDF editor that I have been looking for. It supports drawing and annotating PDF documents. It isn't included in the Ubuntu repositories yet, but there is a Debian/Ubuntu package.
Installation. Get the latest version of the software from here. The filename begins with pdfedit. Then install it with dpkg:
Use. Launch; load a PDF file; annotate.
dpkg -i pdfedit_0.3.1-1~3v1ubuntu0_i386.deb