If you have a Creative Cloud subscription, you can download Creative Suite 6. It can open Pagemaker 6.x files.
Go to the Creative Cloud desktop application. In the list of Apps, there is a menu called Filters & Versions. Choose Previous Versions to download CS6.
You should also search for "Prepare PageMaker files for InDesign" with your favorite search engine. If a PageMaker file is not optimized, it may not convert as well as one that has been prepared.