You cannot SAVE a PNG as a PDF, but you can attempt to print it as a PDF. PDF is a print function, not a save function.
FYI: printing sliced PNG files can be problematic. If you have any slices in your PNG, turn off the visibility of your web layer.
Your question also depends on what you are trying to accomplish. If you want to represent what your png file looks like, use browser display and then screen capture it or print screen.
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I am woman, watch me code.
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Why am I in this hand basket and where are we going?