It depends on what you need the file for. JPG keeps photos small and opens everywhere, which makes it a safe choice for web pages and email. PNG preserves transparency and sharp edges, so it suits logos, icons and screenshots. WebP gives you a smaller file at a similar quality and is supported by all modern browsers. PDF is the right pick when you need to share or print a document rather than a single image. If you are not sure, start with JPG for photos and PNG for graphics with transparency.