I don't think it's a big problem,
because Invalid UTF-8 byte
appear in commented lines .
You can try to replace with
iconv -t 'utf-8'

Anyway, also
ppchtex.tex:             Non-ISO extended-ASCII English text
sort-lan.tex:            Non-ISO extended-ASCII English text
regi-ibm.tex:            Non-ISO extended-ASCII English text, with LF, NEL line terminators

--
luigi