PNG
PNG or Portable Network Graphic format is a graphic file format that uses lossless compression algorithm to store raster images. It uses 2 stage compression methods. It is frequently used as web site images rather than printing as it supports only the RGB color model. So CMYK color images cannot be saved as PNG image.
PDF is a document file format that contains text, images, data etc. This document type is Operating System independent. It is an open standard that compresses a document and vector graphics. It can be viewed in web browsers if the PDF plug-in is installed on the browser.