1. You can install aeroo_reports https://github.com/aeroo, and create a report which outputs .txt
2. You can create a Custom module(with a wizard) that writes and returns a binary field that is a .txt file.

both are time consuming if it is yours first time, number 2 is more flexible, but number 1 is easier.

See this question, in summary:

• You define report with Report Type = HTML.  (or set this type for existing report in Settings/Printing/Reports). HTML ~= TXT
• or
• You define export data as CSV ~= TXT  (Export from More button)