Some text files has line breaks, that might look good on fixed width screens, but can not be used as paragraph in other editing software. The is cause by line breaks.
$ cat input.txt
The computer was a powerful WOPR machine for game simulation.
You can play a nice game of chess or an exciting war game.
This one-liner will replace line breaks with space and then remove any double spaces. It will also make sure, that the file ends with a new line, so it meets the POSIX standard definition of a text file.
$ (tr '\n' ' ' < input.txt | sed 's/ */ /g'; echo) > output.txt
The result is one clean paragraph, that will scale to any screen width or be used in other editing software.
$ cat output.txt
The computer was a powerful WOPR machine for game simulation. You can play a nice game of chess or an exciting war game.