Hello pros,
Does anybody know that how should I do to write a word by UR5? I want to know that is it possible to only use the predefined programs of the teach pendant or we have to write a program basically as a script file (by URsim)?

There are no predefined programs that i know that allow doing this. Unfortunately, you'll have to write your own.

Are you wanting to write random words or predefined words?  If you want to write random words and use Polyscope you could use sub-programs and create one for each letter as well as space, then you could pass into the robot each individual letter of the word and call the appropriate subprogram.  The issue that I see is that there is no use of strings in lists so you would need to pass each letter in individually or convert the letters to a number.  If you convert to numbers that would also allow you to use switch case statement instead of subprograms.

I want to write a predefined word. Actually, I wanted to know that what is the easiest way to write the letters of a word by a UR5 (Since they are much more complicated than normal trajectories to be defined by move L, P or C)

With RoboDK you can make the robot follow a path that you have saved as a DXF (with Adobe Illustrator, Autocad, etc.). That way you can easily make it follow complex paths that are more difficult to program with the teach pendant