There is an extension called PDF import for libre office.Using it you can edit PDF documents in libre office writer.
You can install it from the software-center by searching for pdfimport.
You can install it from the terminal by typing
sudo apt-get install libreoffice-pdfimport
Remember you need Libre-office installed to use this extension. :)
Also there is a similar extension for open office too.
You can install it by searching the same as above in software-center
install it from the terminal by typing
sudo apt-get install openoffice.org-pdfimport