Append header and footer without overlapping the existing content in the pdf using ITEXT API JAVA

Question

In our application, we store PDF in the server and users download it. I need to add the header and footer to the PDF when it is retrieved from the server as byte array and we are using ITEXT API for it.

Here is the code snippet

ByteArrayOutputStream baos = new ByteArrayOutputStream(baNotes.length);
baos.write(baNotes, 0, baNotes.length);
pdfReader = new PdfReader(baNotes);
pdfStamper = new PdfStamper(pdfReader,baos);
Font font = new Font(Font.FontFamily.TIMES_ROMAN, 8, Font.BOLD);
int noteSize = pdfReader.getNumberOfPages();
logger.debug("Retrieving the total number of pages: "+ noteSize);

for(int i=1; i<= noteSize; i++) {
   logger.debug("Adding footer to each notes: "+i );
   PdfContentByte content = pdfStamper.getUnderContent(i);
   Phrase pHeader = new Phrase(fHeader, font);
   Phrase pFooter = new Phrase(fMessage+i, font);
   ColumnText.showTextAligned(content, Element.ALIGN_LEFT,pHeader,60,770, 0);
   ColumnText.showTextAligned(content, Element.ALIGN_LEFT,pFooter,155,20, 0);
}
pdfStamper.close();
bOutput = baos.toByteArray();

If i use the above snippet, in few of the PDF the header and footer is overlapping with the existing content since the co-ordinates are fixed.

I tried many examples to avoid overlapping but in all the examples they are creating the PDF, but in my scenario the PDF is already created and stored in the server. I have also given the link for the ITEXT resource section below for similar question. I also could not able to find from which co-ordinates the actual PDF content is starting.

why does my header overlap my content

Any suggestion/ideas on how to move forward or code snippets are very much appreciated.