Most .dxf files are just text files. If they are being saved with a .txt extension, then it's the way you are saving them. If you just rename the extension to .dxf, then they should open fine in AutoCAD, unless they are saved as .dxf version newer than 2005. I'm running ACA 2018, and there are .dxf versions 2007, 2010, 2013, and 2018. You won't be able to open those.