it snot a pretty work around, but you could get yourself a trial copy of rhino, that works as a great in-betweener for all kinds of formats as Rhino is basically super cool and works exactly as its meant to most of the time!
open the file, then re-export or you can print to pdf at a specified scale.
Illustrator CC cannot open .dxf files, it just crashes and hangs. If you try to open .dwg files instead you get "unrecognized file type" errors. Sometimes it is possible to manipulate linework in pdfs exported from AutoCAD but you will not have proper scale or access to layer information...
If you were to save a .dxf from Illustrator you should have no problem opening that in Illustrator. Sometimes it helps to ask the auto cad person to choose an older format for .dxf or .dwg. Communication with whomever is making that auto cad file is key, if you are doing this yourself then the problem just requires you flipping all the settings until you find what works.
Just use the open command to the default of all readable formats. .dxf is one of the formats.
Choose original size or 100%.
If those are greyed out then you get to play the guessing game size of what until the Auto Cad person was working in before they saved, and that they have dimensions shows on their file.
id ask them to save as dwg if you can, they seem to be more successful for me when im importing stuff.
also check that the file you import is not just positioned away from the view, zoom right out as far as you can, and change view to outline preview, see if anything appears, just incase it is importing with no line colour?