FEATURE REQUEST: PdfPage needs to be able to call CustomPainter and PdfDocument to have the ability to save

[paladinghub]

Hi appreciate such a amazing lib. When opening a PDF file, using pageOverlaysBuilder can effectively display watermarks. However, if the PDF file is opened using other software, the watermark cannot be displayed because lib does not have a save function. I hope that when clicking the save button, PdfPage can use Custom Painter to draw watermarks on the PDF and save it. Thank you very much again