How can i append a pdf into another? Alfresco

How can i append a pdf into another?

I tried using this code, but I am getting java.lang.NullPointerException when i try to getContentInputStream. what am I doing wrong? How can I attach one pdf to another?

PDDocument pdfTarget = null;
InputStream is = null;
InputStream tis = null;
for (ChildAssociationRef file: quotationsFiles) {
 try {
  NodeRef toAppend = file.getChildRef(); //workspace://SpacesStore/11bce382-45bf-4c67-95bc-a65361b323ef

  ContentReader append = getReader(toAppend);
  is = append.getContentInputStream(); // Here iam getting java.lang.NullPointerException 

  NodeRef targetNodeRef = reportFile.getNodeRef();
  ContentReader targetReader = getReader(targetNodeRef);
  tis = targetReader.getContentInputStream();

  String fileName = String.valueOf(serviceRegistry.getNodeService().getProperty(targetNodeRef, ContentModel.PROP_NAME));

  // stream the document in
  pdf = PDDocument.load(is);
  pdfTarget = PDDocument.load(tis);

  // Append the PDFs
  PDFMergerUtility merger = new PDFMergerUtility();
  merger.appendDocument(pdfTarget, pdf);
  merger.setDestinationFileName(fileName);
  merger.mergeDocuments();
 } catch (Exception e) {
  //throw new AlfrescoRuntimeException("IOException", e);
  ColorLogUtil.debug(LOGGER, "IOException Error caused by :" + e);
 }
}

private ContentReader getReader(NodeRef nodeRef) {
 if (serviceRegistry.getNodeService().exists(nodeRef) == false) {
  throw new AlfrescoRuntimeException("NodeRef: " + nodeRef + " does not exist");
 }

 QName typeQName = serviceRegistry.getNodeService().getType(nodeRef);
 if (serviceRegistry.getDictionaryService().isSubClass(typeQName, ContentModel.TYPE_CONTENT) == false) {
  throw new AlfrescoRuntimeException("The selected node is not a content node");
 }

 ContentReader contentReader = serviceRegistry.getContentService().getReader(nodeRef, ContentModel.PROP_CONTENT);

 if (contentReader == null) {
  throw new AlfrescoRuntimeException("The content reader for NodeRef: " + nodeRef + "is null");
 }

 return contentReader;
}

See if this code works for you:

 public NodeRef mergePdfs(List<NodeRef> nodeRefList, String fileName,NodeRef destinationNode) 
            throws FileNotFoundException,FileExistsException,Exception {
        InputStream originalInputStream = null;
        ContentReader reader = null;
        NodeRef newDocNoderef = null;
        PDFMergerUtility PDFmerger = new PDFMergerUtility();
        ByteArrayOutputStream outputstream = new ByteArrayOutputStream();

    try {
        LOGGER.debug("Merging of Doc Started");
        for (NodeRef node : nodeRefList) {
            reader = contentService.getReader(node, ContentModel.PROP_CONTENT);
            originalInputStream = reader.getContentInputStream();
            PDFmerger.addSource(originalInputStream);
        }
        PDFmerger.setDestinationStream(outputstream);
        PDFmerger.mergeDocuments();
        if(originalInputStream!=null) {
            originalInputStream.close();
        }
        newDocNoderef = writeContentToAlfresco(outputstream, nodeRefList, fileName,destinationNode);
        LOGGER.debug("Documents are merged and new pdf is created at "+newDocNoderef);

    } finally {
        if(outputstream!=null)
            outputstream.close();
    }
    return newDocNoderef;
}

public NodeRef writeContentToAlfresco(ByteArrayOutputStream outputstream, List<NodeRef> childRefList,
        String fileName,NodeRef destinationNode) throws FileExistsException,IOException,Exception {
    NodeRef pdf = null;
    Map<QName, Serializable> props =  new HashMap<>();
    Map<Date, NodeRef> dateMap = new HashMap<Date, NodeRef>();
    NodeRef parentNodeRef=null;
    try {
        LOGGER.debug("Upload to Alfresco Started");

        for(NodeRef noderef : childRefList) {
            Date date = (Date) nodeService.getProperty(noderef, ContentModel.PROP_MODIFIED);
            dateMap.put(date, noderef);
        }
        Map<Date, NodeRef> m1 = new TreeMap<Date, NodeRef>(dateMap);
        Map.Entry<Date, NodeRef> entry = m1.entrySet().iterator().next();
        NodeRef finalnodeRef = entry.getValue();

        if(destinationNode!=null) {
            parentNodeRef = destinationNode;
        }else {
            parentNodeRef = nodeService.getPrimaryParent(finalnodeRef).getParentRef();
        }


        QName[] myModelProps = CommonConstants.myModelProps;
        for (QName myModelProp : myModelProps) {
            Serializable object = nodeService.getProperty(finalnodeRef, myModelProp);
            props.put(myModelProp, object);
        }

        FileInfo pdfInfo = fileFolderService.create(parentNodeRef, fileName + ".pdf",
                MyModel.TYPE_CUSTOM_MYMODEL_TYPE);
        pdf = pdfInfo.getNodeRef();

        nodeService.setProperties(pdf,props);
        nodeService.setProperty(pdf, ContentModel.PROP_TITLE,
                nodeService.getProperty(finalnodeRef, ContentModel.PROP_TITLE));
        nodeService.setProperty(pdf, ContentModel.PROP_DESCRIPTION, 
                nodeService.getProperty(finalnodeRef, ContentModel.PROP_DESCRIPTION));
        nodeService.setProperty(pdf,ContentModel.PROP_NAME,fileName + ".pdf");

        ContentWriter writer = contentService.getWriter(pdf, ContentModel.PROP_CONTENT, true);
        writer.setMimetype(MimetypeMap.MIMETYPE_PDF);
        writer.setEncoding("UTF-8");
        writer.putContent(new ByteArrayInputStream(outputstream.toByteArray()));

        LOGGER.debug("Upload to Alfresco Ended");
    }  catch(FileExistsException fee) {
        ExceptionUtils.printRootCauseStackTrace(fee);
        throw new FileExistsException(parentNodeRef, fileName);
    }
    catch (Exception e) {
        ExceptionUtils.printRootCauseStackTrace(e);
        throw new Exception(e);
    } finally {
        if (outputstream != null)
            outputstream.close();
    }

    return pdf;
}