This file is created in response to rather poor documentation [1] for PXD file format consumed by Autodesk Pixlr and other peoducts (e.g., Autodesk SketchBook Pro).
The PXD file, as noted by the few lines in the docs, consists of a header, followed by an array of layers, and then with composite pixel data (on versions >= 3?). All integers within the file are big-endian. The whole file is compressed with zlib.