You need to search the net for how to configure how file types are handled in your preferred browser: https://www.ecosia.org/search?tt=mzl&q=firefox+how+to+change+pdf+behaviour
AFAIK it is not forceable (not a Vanilla Forum limitation) as it depend on browser settings. Some info here https://superuser.com/a/1277820 (if you search for it there are several mentions of the same) Have you tried this plugin? https://open.vanillaforums.com/addon/inlinepdf-plugin