Thanks! I think Photopea can open 99% of PDN files, so I need to find that 1% of files which could cause troubles (and fix the related bugs, to get to 100%).
It is not just a PDN viewer. You will get all the layers (in the layers panel), their names, opacities, blend modes, etc. And you can manipulate it as you would do in Paint.NET (while still in a browser).
Do you think I should add .PDN export? In that case, we would get a complete .PDN editor. The advantage could be, that the UI of Photopea is more similar to Photoshop than to Paint.NET, so you could edit .PDN in a "photoshop way".