PDDestination rawDest = null;
PDAction outlineAction = item.getAction();
if (outlineAction instanceof PDActionGoTo) {
    rawDest = ((PDActionGoTo)outlineAction).getDestination();
}
PDPageDestination pageDest = null;
if (rawDest instanceof PDNamedDestination) {
    PDNamedDestination namedDest = (PDNamedDestination) rawDest;
    PDDocumentNameDictionary namesDict = doc.getDocumentCatalog().getNames();
    if (namesDict != null) {
        PDDestinationNameTreeNode destsTree = namesDict.getDests();
        if (destsTree != null) {
            pageDest = (PDPageDestination)destsTree.getValue(namedDest.getNamedDestination());
        }
    }
} else if (rawDest instanceof PDPageDestination) {
    pageDest = (PDPageDestination) rawDest;
}     

System.out.println(pageDest.findPageNumber());