PageMaker 6.5 was developed for – both 16‑bit and 32‑bit environments. The last versions (6.5 and 7.0) are 32‑bit applications.
Here’s a useful write-up about with special attention to the "64-bit" question. Adobe PageMaker 6.5: A Retro DTP Powerhouse – And the Truth About “64-Bit” Introduction Adobe PageMaker 6.5 is a legendary desktop publishing (DTP) application originally released in the late 1990s. It was widely used for creating brochures, newsletters, flyers, books, and newspapers. While it has since been discontinued (replaced by Adobe InDesign), many older publishing houses, legal offices, and archival projects still rely on PageMaker 6.5 for legacy document management. The “64-Bit” Clarification – Very Important There is no native 64-bit version of Adobe PageMaker 6.5. Adobe Pagemaker 6.5 64 Bit