PdfWriter not appending data

Question

lowagie PdfWriter is not appending data but overwriting it whenever writeData is called. How can we append data?

void writeData(List<org.bson.Document> item){
boolean isExistingFile = Files.exists(Paths.get(filePath));
File file = new File(filePath);
PdfPTable table = null;
if (!isExistingFile) {
    writer = PdfWriter.getInstance(doc, new FileOutputStream(file, true));
    writer.setPageEvent(reportPdfWriter);
    table = new PdfPTable(headerLength);
    // table cell added for each header
        {   table.addCell(header); }
}
for (org.bson.Document d : item) {
    Iterator<Object> itr = d.values().iterator();
    while (itr.hasNext()) {
        Object obj = itr.next();
        table.addCell(String.valueOf(obj));
    }
}
doc.open();
doc.add(table);
}