Postgres \copy command into single column without delimiter
You cannot use COPY or \copy if there is no character which is certain not to appear in the data. You will either need to pick a character to act as the delimiter, then escape or quote it and the escape/quote character in the input file (perhaps using PROGRAM).  Or, not use COPY or \copy, but rather use INSERT instead.

It isn't clear from your question if your files have newlines as record separators or if every file is one record and internal newlines are literal.  If the latter you do use COPY or \copy, you will have to escape/quote that as well.