Download Entire iMessage History with One Person as Plain Text

I am doing some archiving after a loss and would like to preserve a chat thread with one person in plain text or pdf (not .HTML by date). The thread goes back 2.5years, so there is a lot. I have accessed the ~/Library/Messages folder but there is no good way to get the thread/messages into a format where i can make one large file of all of them. Any ideas?