One great thing about Google Docs is that you can conveniently add a PDF to an existing document in four distinct ways.

Open Google Docs. If you're not logged in to your Google account, you'll need to do that now. Select File > Download > PDF Document (.pdf). Save the PDF to your computer.

This article explains how to convert a Google Doc to PDF. The Google Docs website provides a few ways to do this depending on what you want to do with the PDF after it's been converted.