pm6 to indd conversion is probably searched by users looking for a way to convert old Adobe PageMaker documents to InDesign format or at least to import PageMaker files to Indesign. Everything points to the fact that InDesign does indeed support certain PageMaker files, so this is probably the way how to do it. But the version CS6 was the last one that offered this and the latest versions of InDesign. And even in the old versions, it required significant manual editing of the original source.
Or perhaps you can reach out to old PageMaker forum and ask someone to export your old files to some exchange format that can be transferred to InDesign.