There are two ways to export a custom report once it has been run.
On the upper right you can click Print which will export the report to PDF. Email will also be pdf.
You can also click on the blue down arrow just above each view within the report to export that view to a .csv file.
There is no option to export to .txt.
However, The csv file can be opened in notepad.
The file extension means 'comma separator values' so in
the text file it will just list all of the information
separated by a comma.