Create PDF files is a very common operation. Modify them is a bit less common but, anyway, a very requested feature.
LibreOffice exports documents as PDF by default. You simply click File -> Export in PDF format.
But, if you want, you can also import a PDF file into Writer to modify it. All you need is an extension called pdf-import. To install it, simply execute following commands in your terminal:
$ sudo apt-get install libreoffice-pdfimport
At the end of the process, you will be able to open PDF files directly with LibreOffice Writer by right clicking on it and selecting “Open with Writer”.