PDPage page = (PDPage)allPages.get( i );
PDRectangle pageSize = page.findMediaBox();
float stringWidth = font.getStringWidth( message );
float centeredPosition = (pageSize.getWidth() - (stringWidth*fontSize)/1000f)/2f;
PDPageContentStream contentStream = new PDPageContentStream(doc, page, true, true);
contentStream.beginText();
contentStream.setFont( font, fontSize );
contentStream.moveTextPositionByAmount( centeredPosition, 30 );
contentStream.drawString( message );
contentStream.endText();
contentStream.close();