Works perfectly fine here. Windows 11 with paint.net 5.0.2.
I notice that Windows 11 from time to time asks for confirmation of file-type-associations. It did so with *.pdn-files, too.
I'v got PDN 4.0.9 on Win7 64 bit installed. And I fail to open that tiff with PDN, too.
What makes me wonder is that I can open it with the windows-built-in Photo-Viewer.
I saw on http://optipng.sourceforge.net/ that opting.exe 0.7.4 is the newest version.
I realized that it can be used with the dll provided in the first posting of this thread.
Or are there features missing when the dll is not updated?