Adobe Pagemaker was discontinued 2004 but a lot of users kept using for couple of years later and many of them still have decent cache of documents and publications saved as .pmd files.
Not even the latest version of its successor, InDesign, can open old PageMaker files. Actually, PageMaker files are not supported in any other program, so your only viable options is to either find old copy of PageMaker or convert the files to something else.
If you find some old copy of PageMaker (for example on some of sites that proved old software) or you already have the program installed, you just now need to install virtual PDF printer and simply print your file .pmd file as .pdf document.
If you are MacOS user without access to desktop PC, you will need to use Windows virtual machine to install old PageMaker and convert pmd to pdf on Mac.
PlanMaker program also uses PMD extension for its default documents, these are not related to Pagemaker, but can also be exported to PDF within the program or with a virtual PDF printer.