Infix - cool application, which allow to create protected PDF files!
But I haven't found how to add my own water sign for PDF.
You can add your own watermark to a pdf. You can do this by selecting "Watermark->Add..." under the "Document" menu item.
Adding watermarks using Infix is explained on page 145 of the Infix User Manual. You can access the Infix User Manual via the "Help" menu item.