The reading for this word is just the jukugo, but the reading for 文 is a little bit modified. Instead of the reading もん, which you learned with the kanji, it's just も. You can remember this because you're shortening this word in a way as well. The "writing letter" is shortened to "letter." So, think of もん being shortened to も.
Make this letter yellow, please.
Michael took eight hours to write a letter to Koichi in hieroglyphics.
It looks like that email from you contained corrupted characters, what did you write?