Adobe Pagemaker is a discontinued product, however a lot of users that were actively using it still have plenty of documents or publications saved as a .pm4 files (probably version 4).
These files cannot be opened in anything else, so the only viable way is to convert / export them to something else. Obvious conversion here is export to PDF (pm4 to pdf conversion), which can be done either through export function in PageMaker or better through a virtual PDF printer installed so you have the latest version of PDF format.