I've managed to automate the watermarking of PDF documents i.e in c# i've hooked into an the adobe dll, and calling an Adobe menu action that applies a specific watermark to all pages. The code and automation of watermarking works well, but the watermarking only works if the PC its running on is not locked or in sceen saver mode. It seems the PDF has to be active (not necessarily focused) to apply the watermark.
Also the watermark cannot be applied as a background process. Does anyone know a way of applying PDF watermarks even if the PC is locked/screen saver running etc or know any background methods?