Assuming doc is your instance of org.w3c.dom.Document:

TransformerFactory tf = TransformerFactory.newInstance();
Transformer transformer = tf.newTransformer();
transformer.setOutputProperty(OutputKeys.OMIT_XML_DECLARATION, "yes");
StringWriter writer = new StringWriter();
transformer.transform(new DOMSource(doc), new StreamResult(writer));
String output = writer.getBuffer().toString().replaceAll("\n|\r", "");

you can try

CopyDocumentBuilder db = DocumentBuilderFactory.newInstance().newDocumentBuilder();
InputSource is = new InputSource();
is.setCharacterStream(new StringReader("<root><node1></node1></root>"));

Document doc = db.parse(is);

refer this http://www.java2s.com/Code/Java/XML/ParseanXMLstringUsingDOMandaStringReader.htm

Relies on DOM Level3 Load/Save:

public String getStringFromDoc(org.w3c.dom.Document doc)    {
    DOMImplementationLS domImplementation = (DOMImplementationLS) doc.getImplementation();
    LSSerializer lsSerializer = domImplementation.createLSSerializer();
    return lsSerializer.writeToString(doc);   
}

Firstly, as others have stated, you shouldn't use any XML processing. Just read the file as a text file.

Secondly, your umlaut characters showing up as '�' is due to an incorrect charset (encoding) being used. The charset error may be in your code, or it may be the XML file.

The original XML file contains encoding="windows-1252", but it's unusual for XML to be encoded in anything other than UTF-8, so I suspect the file is really a UTF-8 file and the encoding it claims to use is not correct.

Try forcing UTF-8 when reading the file. It's good practice, regardless, to specify the charset when converting bytes to text:

String xml = new String(
    Files.readAllBytes(xmlFile.toPath(), StandardCharsets.UTF_8));