With dos2unix abc.txt you have successfully converted the DOS text file to a Unix text file.
The file still contains a newline character. This is a totally valid thing to have in a text file. It's simply an empty line of text.
If you want to totally truncate the file, then
>abc.txt in the shell would do that.
To remove empty lines at the end of a file, see "https://unix.stackexchange.com/questions/81685/how-to-remove-multiple-newlines-at-eof".