PRN files are most easily handled by a PC-based application. It does appear that the Mitsubishi F series MELSEC program command set includes an ASCII PRINT function (FNC code 77). I am not very familiar with the newer Mitsubishi PLC's, but I would imagine that someone has posted a solution on their forum or support site.
Here's how to try out printing to the Zebra printer:
Match the serial communications between your PLC and the printer. Either change the settings on the PLC to match the printer or the printer settings to match the PLC.
Default Zebra settings are typically: 9600 baud, 8 data bits, 1 stop bit, no parity, and XON/XOFF. PROTOCOL should be set to NONE.
Depending on the way your PLC serial port is pinned, you may need a null modem cable.
I also found this on a tech forum: Try sending the following ZPL code from your PLC to the printer as a test: