Both .pxz and .pxd files come from the web-based graphics editor app called Pixlr and this is actually the only way how you can ever convert or resave pxz to pxd. As a free user you can do limited number of saves per day, but that should be enough for few conversions. In any case, no other software supports Pixlr formats, so it's not like you have alternative.
Updated: October 23, 2023