If you're using Evince (aka Document Viewer) which is the default PDF viewer in Ubuntu, you might have seen that sometimes, some text is missing in PDF documents (especially Japanese fonts but not only).
Here's the solution: install poppler-data package.
open a terminal and enter the following command (or use Synaptics):
sudo apt-get install poppler-data
Press enter and that's all (note you that you must have multiverse repository enabled).
Now you can enjoy the full content of your PDF documents!!! :-D