As above it is not normal practice to have been given the installer manual for a professionally installed alarm system.
Also as above, if a system of this age 'goes wrong', the installer manual is unlikely to be of any use beyond disabling it pending a replacement system. Unless you're a fan of 30 year old washing machines, microwaves, TVs, light bulbs, cars... etc... it seems odd to be wanting to keep a 30 year old alarm going.