PDF stands for "portable document format". Essentially, the format is used when you need to save files that cannot be modified but still need to be easily shared and printed. Today almost everyone has a version of Adobe Reader or other program on their computer that can read a PDF file.
How to open a PDF file

You can open and view PDF files with a large number of applications, most notably, Adobe Acrobat Reader (multiplatform) and Adobe Acrobat (multiplatform), which can also edit them. Nitro Pro (Windows) and Smile PDFpenPro (macOS) are other applications that provide PDF viewing and editing capabilities.

Other options for viewing PDF files include Microsoft Word (Windows and macOS), Apple Preview (bundled with macOS), Foxit Reader, and web browsers, such as Google Chrome (multiplatform) and Microsoft Edge (multiplatform), which includes annotation capabilities.

On mobile devices, you can view PDF files with Adobe Reader (Android and iOS), Apple Books (iOS), Apple Pages (iOS), Google Play Books (Android), and in web browsers, such as Chrome and Safari.