A lot depends on what format you want for the mainframe file. A simple format that can be created by the Save As… menu choice is to select the CSV format and say “Yes” to the two warning prompts that follow. The result will be a text file in the .csv format. This will have the column labels in the first line, followed by the data in the following lines.
If you want a fixed column format, save it the printer format (.prn). You can view it in Notepad to confirm the result. This format will size the columns according to the longest content in each column. Again, the first line will be the column titles, a direct copy of the first line, or just the data if there are no column titles.