Implemented as a category on NSDictionary.

CaseInsensitiveDictionary.m

static Boolean PSPDFCaseInsensitiveEqualCallback(const void *a, const void *b) {
    id objA = (__bridge id)a, objB = (__bridge id)b;
    Boolean ret = FALSE;
    if ([objA isKindOfClass:NSString.class] && [objB isKindOfClass:NSString.class]) {
        ret = ([objA compare:objB options:NSCaseInsensitiveSearch|NSLiteralSearch] == NSOrderedSame);
    }else {
        ret = [objA isEqual:objB];
    }
    return ret;
}

static CFHashCode PSPDFCaseInsensitiveHashCallback(const void *value) {
    id idValue = (__bridge id)value;
    CFHashCode ret = 0;
    if ([idValue isKindOfClass:NSString.class]) {
        ret = [[idValue lowercaseString] hash];
    }else {
        ret = [(NSObject *)idValue hash];
    }
    return ret;
}

- (NSDictionary *)pspdf_caseInsensitiveDictionary {
    CFDictionaryKeyCallBacks keyCallbacks = kCFCopyStringDictionaryKeyCallBacks;
    keyCallbacks.equal = PSPDFCaseInsensitiveEqualCallback;
    keyCallbacks.hash  = PSPDFCaseInsensitiveHashCallback;

    void *keys = NULL, *values = NULL;
    CFIndex count = CFDictionaryGetCount((CFDictionaryRef)self);
    if (count) {
        keys   = malloc(count * sizeof(void *));
        values = malloc(count * sizeof(void *));
        if (!keys || !values) {
            free(keys), free(values);
            return self;
        }
        CFDictionaryGetKeysAndValues((CFDictionaryRef)self, keys, values);
    }

    CFDictionaryRef caseInsensitiveDict = CFDictionaryCreate(kCFAllocatorDefault, keys, values, count, &keyCallbacks, &kCFTypeDictionaryValueCallBacks);

    free(keys);
    free(values);

    return CFBridgingRelease(caseInsensitiveDict);
}