I have had a few customers recently having issues using SPLTOOL to convert spooled files to PDF and adding an overlay, and/or concatenating another PDF to the created document and running into this error:

java.io.IOException: The specified device does not exist.
	at sun.nio.ch.FileChannelImpl.map(FileChannelImpl.java:923)
	at com.lowagie.text.pdf.MappedRandomAccessFile.init(Unknown Source)
	at com.lowagie.text.pdf.MappedRandomAccessFile.<init>(Unknown Source)
	at com.lowagie.text.pdf.RandomAccessFileOrArray.<init>(Unknown Source)
	at com.lowagie.text.pdf.RandomAccessFileOrArray.<init>(Unknown Source)
	at com.lowagie.text.pdf.PRTokeniser.<init>(Unknown Source)
	at com.lowagie.text.pdf.PdfReader.<init>(Unknown Source)
	at com.lowagie.text.pdf.PdfReader.<init>(Unknown Source)
	at watermark_pdf.main(watermark_pdf.java:31)

Now, the problem for this is normally that the folder(s) in use are being journaled.  For some reason this doesn't play nice with Java on the IBM i, so you'll need to make sure directories used to create PDFs, store overlays, etc, are not journaled on your system.