Try adding the a Where for DocumentMenuItemHideInNavigation.
var documents = tree.SelectNodes()
.Types("CMS.MenuItem", "cpabc.landing")
.Path(Path, PathTypeEnum.Children)
.NestingLevel(1)
.OnSite("CpaBc")
.Published(true)
.Where("DocumentMenuItemHideInNavigation", CMS.DataEngine.QueryOperator.Equals, "0");