Xournal is an application for note taking or sketching and even though its page says it's a tool similar to Microsoft Windows Journal, Jarnal, Gournal, and NoteLab, I'd also add Foxit Reader to the list.
That's because even though its Sourceforge page doesn't mention this, Xournal can also be used as a lightweight PDF viewer and further more, it can be used to highlight text in PDF documents, make annotations and so on. However, please note that Xournal is not a PDF editor!
To use Xournal to highlight or add text to a PDF file, select File > Annotate PDF (you can also chose the usual File > Open, but make sure you then select to view all files, not just Xournal files) and select the PDF you want to annotate or highlight text.
Xournal cannot save a file in PDF format but you can easily select File > Export to PDF to achieve something similar so it's a viable solution for those who are looking for a reliable tool to annotate/highlight text in PDF files on Linux.
Xournal is available in the Ubuntu repositories and you can install it by clicking the link below (if you're reading this in a feed reader, the button won't work - you'll have to visit the post on WebUpd8 to use it):
If you prefer to install Xournal from the command line, open a terminal and copy/paste the following command:
sudo apt-get install xournal