Two different types of pmd to pdf conversion exist, both dealing with completely different kind of export of document files to PDF format.
Adobe Pagemaker was discontinued 2004, but a lot of users kept using it for couple of years later and many still have documents and publications saved as .pmd files. Unfortunately, not even the latest version of its successor product, InDesign, can open old PageMaker .pmd files. Considering the overall limited support for PageMaker, your options for any pmd conversion are pretty limited. The only way would be to find old copy of PageMaker, install virtual PDF printer (if you do not have it already) and export pmd to pdf there.
PlanMaker program also uses .pmd files for documents, but it's completely different format. You will need to install PlanMaker to convert pmd to pdf of this type.Open PMD file Open PDF file
One of the first desktop publishing programs on market
Mac version of once popular publishing program
A handy spreadsheet editor from SoftMaker Office suite