Hi,
I refer to this post - IdentityNow Correlation Rule - Identity Security Cloud (ISC) / ISC Discussion and Questions - SailPoint Developer Community
As per this, can you try to type cast the returned List to sailpoint.rule.identity type of list and see if it works.
List<sailpoint.rule.Identity> identities = idn.findIdentitiesBySearchableIdentityAttribute(“documento”, EQUALS_FILTER, documento, “documento”);