A PMLZ file is a complete ebook - a ZIP file containing one PML file and any associated image files.
PML, Palm markup language, is the source file format for eReader format. Once compiled into a PDB it can be read my eReader compatible readers. However, the source file is really a set up files which includes the markup Text and all of the images. Zipping this set of files creates the PMLZ file. It can be unzipped as needed when changes are needed.