Here is a sample java program showing how to add a redaction annotation to a PDF document, export them as xfdf and then re-import them into another PDF document using Qoppa library jPDFProcess.

// create a new PDF and append a page to it
PDFDocument doc1 = new PDFDocument();
PDFPage page1 = doc1.appendNewPage(72 * 8.5, 72 * 11);
// create redaction annotation and set properties
Redaction annotation = doc1.getAnnotationFactory().createRedaction("");
// set annotation bounds
annotation.setRectangle(new Rectangle2D.Double(100, 100, 100, 100));
// annotation border color (Red)
// annotation opacity
// redaction color (Gray)
// set overlay text color (Blue) 
// set overlay font police and size 
annotation.setOverlayFont("Times-Roman", 8); 
// set overlay text 
annotation.setOverlayText("[NEW TERM]"); 
// set overlay repeat 
// add redaction annotation to page 1 
// export annotations to an xfdf file 
doc1.exportAnnotsAsXFDF("C:/support/redaction.xfdf", null); 
// create a new PDF and append a page to it 
PDFDocument doc2 = new PDFDocument(); 
doc2.appendNewPage(72 * 8.5, 72 * 11); 
// import annotations from FDF into PDF 
// save the PDF 

Here are the attributes in the XFDF files specific to the redaction annotation:
coords Optional
interior-color Optional
overlay-text Optional
overlay-text-repeat Optional
justification Optional