Create a printer file with DEVTYPE(*USERASCII) specified. The *Outq and any other miscellaneous parms can be filled in.
Create an FTP script (or session) with BINary and CD PrtfLib (the library containing the *PRTF)and a PUT from the IFS to the printer file.
The reason for the CD PrtfLib (and this seems to be a key element) is that you should not fully qualify the target of the PUT. If you do qualify the *PRTF, FTP behaves differently. The result is that the *PRTF gets overlaid with the ASCII data.
PUT /MYFOLDER/MyStreamFile MyPDFPrtf