PDF is a really cool thing that helps us share documents with each other. It's like a special way of making a copy of a piece of paper with words and pictures on it, and then sending that copy through the computer instead of giving someone the actual paper.
Sometimes we might want to send a piece of paper to someone, but it's too hard to get to them in person. So instead, we can put the paper into a machine that scans it and turns it into a digital file. This file is called a PDF.
Once the paper is turned into a PDF, it doesn't change the way it looks, but now it can be opened on a computer or a phone. That way, people can read it, see the pictures, and even print it out on their own piece of paper if they want.
PDFs are a very popular way to share important things like your homework, your resume, or even a book you wrote, because it's a safe and easy way to get it from one place to another without losing the way it was meant to look.