Paint.NET graphics editor uses its own, special .pdn files to save edited images instead some of the common formats. Most users however, prefer to save images to some common format like JPEG (.jpg, .jpeg), or PNG (.png), hence they look for pdn to png conversion for example.
As far as we know, .pdn images can only be worked upon in the original tool, so you will have to download it (it's free) and save it to something else. But perhaps other programs introduced support for this particular graphics format as well.Open PDN file Open PNG file
A free photo and image editor