I could be showing my age here, but the document could be in EBCDIC format rather than ASCII because it was a bigger character set by 1 bit and thus favoured by word processors of the time for that extra capability.
It was also very common from when computers were invented to incorporate "magic numbers" inside a file, which are really just the first few bytes of the file that are unique to a particular file type. Try googling yours if you think there is one.
One trick I do on a Linux system is run a command called "strings" on data files to quickly get a sample of the text that is in them.
Otherwise perhaps try emacs.
Emacs users are quite rare.