I think I have found an anwer...

I opened the underlying .RTF letter in Word and confirmed the paper size was set to Letter. Several of the letters were set to A4 paper size. 

The letters are located in the \ProWin25\common folder. There are several letters, so I just opened each one. From the MS-Word menu, select Layout | Size | Letter. Then save the .RTF file. (The standard client letter is "fdiltr.RTF".)

I restarted ProSeries, then when I printed the return to PDF, the margins returned to normal, and the letter printed just fine. 

I hope this helps someone else!