Hewlett Packard (HP) was formed in 1939 and started developing test equipment. Over the next 61 years HP manufactured a very wide range of test equipment. In 2000 the business became Agilent Technologies. Fortunately there is a lot of HP information available on the web.
For details of the HP history see:
My particular interest is in the older HP test equipment, typically manufactured in the 1970's and 1980's. This is because it has become affordable on eBay; still offers good performance; plus maintenance is still practicable largely due to the availability of HP Operation & Service Manuals, spare parts and knowledgeable online forums.
With thanks to Agilent and several other websites, I have been able to download various technical information that has helped to restore and understand my test equipment.
Although this information is currently available on the web, I like to have anything relevant to my interest area available for offline access on my computer. The information is an excellent learning aid, the only problem is having enough time to study it all and clever enough to understand it!