Notes does have it’s own set of ‘viewers’ built in. When you view an attachment the Notes client uses those ‘sub-programs’ or plug ins for lack of the proper term.
When you open an attachment, however, it should use the native operating system default program… i.e. for a document MS Word might launch.
I find it curious that your clients are not doing that. Perhaps there is a property setting I am unaware of somewhere in their R5 client. You might ask them if they are using the ‘View’ button, and suggest they use ‘Open’ or [forgive me I have been on 6.5 for a while] whatever the open attachment option is and use that instead.
Good luck with it.