One possible cause, You are FTPing in binary mode so no character set conversion takes place. If your i5 has CCSID = 65535 (the default) then once on the AIX LPAR, the data is binary, not character. If your file is in CCSID 65535, you need to specify the character set conversion in the CPYTOIMPF cmd.
It seems odd… If the file is created and written as *PCASCII, even if the system is set with QCCSID=65535, then a BINARY transfer should transfer a *PCASCII file without any conversion. Which should be okay.
It’s a *PCASCII file on the source system and the target system should see the exact same unconverted (BINARY) bit patterns. If it starts as ASCII and you want it to end up as ASCII, there should be no reason for code conversion.
But it seems there’s a good chance that something else is involved. We’re told what happens, but we’re not shown what happens. We can’t tell if there’s simply an obscure detail that was missed or a misunderstanding is involved.
<b>Show us</b> what happens. Copy/paste file attributes here. Copy/paste actual commands that are run. Tell us operating system versions/releases. We need to actually see things rather than just read about them (which <b>always</b> leaves out potentially relevant details.)
Show us what happens. We can probably ask the next questions from there.