PNG is a lossless image format. That makes it a strong choice for screenshots, app mockups, logos, charts, and anything with crisp text or flat colors. JPG can blur those edges with compression artifacts.
Converting PNG to PDF is useful when you need to send screenshots as a document, combine several diagrams into one file, or preserve a transparent logo page in a portable format.
Use PNG to PDF when quality and sharp edges matter. If the images are ordinary photos and file size is the priority, JPG-to-PDF is usually smaller.