PDF is a "portable document format" that can be opened by many different programs. When you send someone a Word document, they need a copy of Microsoft Word to open it. That means, if they haven't paid for Microsoft Word, they can't open it. The same thing happens with Pages documents. When you use PDF, anyone can open it using whatever program they have on their computers.
You use PDFs in three ways:
My favorite PDF tool is KAMI. It's a Chrome extension so it works on Macs and Windows.
Questions about how to use PDFs? Ask Mrs. Hanson.