SLPR stands for Streaming Line PRinter.
SLPR.PDR is a streaming port driver used to connect your computer to a printer or print server via a TCP/IP connection. The driver uses Line Printer Daemon (LPD) Protocol as specified in RFC 1179. This protocol is an almost universal way to send print data to printers and servers. It is meant to be used as a alternative to the \PIPE\LPD0 (henceforth LPD0) port driver which is shipped with OS/2.