I could able to write a function based on your suggestions thanks for that. But I receive a nullpointer exception it may be because I am missing something or doing it wrong. Basicallly I am not good in java wrote a code just based on your inputs and able to compile the function correctly in PEGA. Please take a look at my code.

try {
//String parameter 'base64decoded' will contain base64 decoded TIFF image string at runtime
byte[] tiffBytes=base64decoded.getBytes();
RandomAccessFileOrArray myTiffFile=new RandomAccessFileOrArray(tiffBytes);
int numberOfPages=TiffImage.getNumberOfPages(myTiffFile);
Document TifftoPDF=new Document();
ByteArrayOutputStream bOutput = new ByteArrayOutputStream();
        PdfWriter.getInstance(TifftoPDF, bOutput);
        TifftoPDF.open();
        //Run a for loop to extract images from Tiff file
        //into a Image object and add to PDF recursively
        for(int i=1;i<=numberOfPages;i++){
            Image tempImage=TiffImage.getTiffImage(myTiffFile, i);
            TifftoPDF.add(tempImage);
        }
        TifftoPDF.close();
byte[] pdfoutputinbytes=bOutput.toByteArray();
String finalPDF=Base64Util.encodeToString(pdfoutputinbytes);
return finalPDF;
}

catch (DocumentException e) {
                oLog.error(e);
return "ERROR";
       
            }

Please give your comments.