To download SNMP MIB files, refer to https://www.paloaltonetworks.com/documentation/misc/snmp-mibs.html.
https://knowledgebase.paloaltonetworks.com/KCSArticleDetail?id=kA10g000000ClNnCAK&lang=en_US&refURL=http%3A%2F%2Fknowledgebase.paloaltonetworks.com%2FKCSArticleDetail