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.
.PDF can be createdfrom Screen Sketch, Microsoft Draw, (or any application) by printing the .png file with "Microsoft Print to .pdf" in the print menu. Unfortunately, Microsoft has not provided a Print function directly in Windows Ink. But you can take a screenshot and print that as an alternative to saving a .png then printing it.