Googled QTLPD and found the following Q&A on Midrange site.
The QTLPD* jobs are Line Printer Daemon jobs — LPD. These jobs listen for connections from other systems that want to send spooled files to it for printing (or simply for retention on an output queue.) Any system, even PCs or your AS/400 itself might be the source.
E.g., running SNDTCPSPLF can send a copy of a spooled file to a different outq on your system. LPD is what receives the spooled file and places it on the named outq.
If connections are being made that are unexpected, review connections in iSeries Navigator or with NETSTAT *CNN to see the remote address.
See command CHGLPDA to review or change startup attributes, particularly the number of jobs to start when the server is started. The server can be auto-started when TCP/IP starts or it can be started with STRTCPSVR *LPD. End the server with ENDTCPSVR *LPD.