The only problem I can think of wrt. plain text files is that the server supplies a Content-Type: text/plain; charset=encoding with different encoding than you use on your local machine. (Typically, the server provides UTF-8, and you use e.g. Windows Western European aka win-1252 aka Codepage 1252.)
The effect is that all ASCII characters are fine, but non-ascii characters are mangled.
One other quirk is that you might use a text editor that requires Windows CR-LF newline convention, whereas Linux, BSDs, Macs use LF-only.
There is no "fix" nor "error".
What you need to do, is view the file in your browser, and change the encoding to what the server used.
(Browser developers are stupid, and don't have any way of setting local text files' encoding to UTF-8, "because it is not a legacy encoding and is therefore unsuitable as a default". Yeah, it is one of my pet peeves.)
Because you intend to do an archival, I recommend against recoding the files with e.g. iconv.