If it's a plain text file in .txt format, there aren't too many things you can do. Especially considering that you would need to lose 1/3rd of the size. If it's a formatted text file in .doc format for instance, convert it to .txt.
On a side note:
"In an uncompressed format, each character in a text document is encoded using an 8-digit series of 1s and 0s. 8 digits is enough to encode every character in the English language, plus many other useful characters. But 8 digits is also a lot, and surely there must be a way to encode the same number of characters with fewer digits.
In 1952, David Huffman invented Huffman coding. Huffman coding involves sorting every character in a text document by how often they appear, and then creating a tree with each character on it. A sequence of binary digits will point to a specific place on the tree, and the tree is deliberately constructed so that there's no possibility of mistaking one character for another."
The issue is that if you encode your .txt according to this and the file size gets smaller, it will be unreadable until you decode it into a readable text format.