Well they are different. A png file is a format used for higher resolution images and called raster images. PDF's are vector images
Raster: Unlike a vector image, raster images are resolution dependent, meaning they exist at one size. When you transform a raster image, you stretch the pixels themselves, which can result in a “pixelated” or blurry image.
Vector: Unlike raster images, vector images are resolution independent. When you shrink or enlarge a vector image, your shapes get larger, but you won’t lose any detail or get any pixelation. Because your image will always render identically, no matter the size, there is no such thing as a lossy or lossless vector image type.