Is there a way to send a form to a .TXT file instead of to a physical printer?

Install a "Generic Text-Only" Windows printer specifying the port "FILE". Go to the Forms Menu and set the printer for Mode 1 to that printer (Options>Printer Settings). Now, print the 1099INT form selecting Mode 1, Windows will ask for the file name you want to print to, specify the file name and use the extension .TXT.