All data on the computer is binary. Text is just a way of interpreting binary data. You arbitrarily set a number to a real world language alphabet.
Example, I decide the number 1 means a. Or maybe the number 400 means a. Or maybe 666 means a. It's arbitrary, yes. The most common in usage is ASCII though.
A text file is thus a binary file that has been interpreted as text, following some arbitrary mapping of numbers to letters.