For niche uses, you might encounter PMD files in these environments:
Adobe InDesign is the industry standard and the official successor to PageMaker. It can open PMD files directly, though you may need to fix some formatting after conversion. Get Adobe InDesign The Free Alternative: LibreOffice pmd file opener link
If you successfully open your PMD file, do not save it back to .pmd. Immediately export it to a universal format like PDF (for design) or XLSX (for spreadsheets). The PMD format is a dying standard, and you may not be able to open it again in ten years. For niche uses, you might encounter PMD files