A pdf file is a "portable document format" file created by Adobe. It can be read by any computer that has Adobe Reader (and all do nowadays). It allows files to be read by different computers that don't necessarily have the same software, or the exact version, that the original file was created in.
Adobe has tried to call it "Acrobat" and has even tried to use the term "Reader", but over the years, the term pdf (which is the extension) has stuck. So if someone asks to you send them a pdf, or if you receive a pdf, that's all it means. If someone sends you a pdf file, just click on it, and Adobe Reader will open it up. No, you can't make any changes to it, but at least you can look at it, which was a pretty big thing when it was first invented back in the '90s.
To create a pdf file (I won't go into detail here), your software program must have an option to "save as pdf" (which most do nowadays). If it doesn't, you can distill (yes, that's what they call it) it in Adobe Acrobat Pro.
Back in the '90s I made fun of Adobe for trying to create a file format that any computer could read. But they succeeded beautifully. How about that?