NET-SNMP-PERIODIC-NOTIFY-MIB: View SNMP OID List / Download MIB

Download standard MIB format if you are planning to load a MIB file into some system (OS, Zabbix, PRTG ...) or view it with a MIB browser. CSV is more suitable for analyzing and viewing OID' and other MIB objects in excel. JSON and YAML formats are usually used in programing even though some systems can use MIB in YAML format (like Logstash).
Keep in mind that standard MIB files can be successfully loaded by systems and programs only if all the required MIB's from the "Imports" section are already loaded.
The tree-like SNMP object navigator requires no explanations because it is very simple to use. And if you stumbled on this MIB from Google note that you can always go back to the home page if you need to perform another MIB or OID lookup.