How to make text file created in Ubuntu compatible with Windows Notepad?

Unlike Unix where a new line is represented by a LF character we need a combination CR/LF for files in DOS/Windows:

We can let Gedit save text documents with Windows-style line endings in the File -> Save as dialog.

To make nano write text files in DOS format we have to run it with the following command line option ¹:

nano --dos <filename>

Vim can convert files from Unix to DOS format with the following control sequences ²:

:update
:e ++ff=dos
:w

To set the buffer coding to DOS style issue Meta + x :

set-buffer-file-coding-system utf-8-dos

Converting Unix-style line termination to Windows-style and back

I'm not sure if it's installed by default. Try this to be sure

sudo apt-get install dos2unix

You can convert the files with this to Windows-style

unix2dos myFileFromUbuntu.txt

And with this back to Unix-style

dos2unix myFileFromWindows.txt

Changing them back is often unnecessary. You will find, that the standard text editors handle Windows-style line termination just fine.