PMD files are binary morph data, not compatible with versions below 6 as Cage says. To confirm your supposition, you can incorporate them by unchecking "Use external binary morph targets" and re-saving, whereupon you can delete the PMD file. Compressed file types won't work in Pro Pack or earlier, and compressed OBJ (OBZ) won't work in P5 either. If you uncheck the appropriate preference you can save the normal version. There's also a script in your Python window which will un-compress a file for you. Since disk space is cheap, I don't use external morphs or compression by default; it's easier to use Poser file editors on the "vanilla" P4 compatible files, and possibly easier to recover from file corruption should it occur.