How to make letters fall from sky like snow?

Here's a way you could do it. Just create a script named LetterRain.cs and add this to it, then add that to a gameobject in your scene. This is far from perfect, but it works and does what I believe you are looking for. You can tweak the startPosMax and starPosMin, and probably set those positions based on the current camera location.

 using System.Collections;
 using System.Collections.Generic;
 using UnityEngine;
 
 public class LetterRain : MonoBehaviour {
  
 
     private List<GameObject> letterPool; // Creating a pool of objects is the most memory efficient way to handle things like this. No new memory will be allocated during runtime.
     private List<GameObject> lettersAlive; //A list of the letters currently alive. 
     private int poolSize; //Poolsize, will be set based on lenght of poem.
     public string poem = "This is a very nice litle poem. It is about letters raining";
     public Vector3 startPosMax = new Vector3(10, 10, 3); 
     public Vector3 startPosMin = new Vector3(-10, 10, -3);
     public float lettersPerSecond = 10; //How many letters per second. Max is one per frame, see how it's implemented in update.
     public float aliveTime = 2f; //How long each letter should live.
 
     private float timer = 0; //Timer to keep track of when to add new letters.
 
     // Use this for initialization
     void Start () {
         CreatePool();
     }
 
     private void CreatePool(){
         poolSize = poem.Length;
         letterPool = new List<GameObject>();
         lettersAlive = new List<GameObject>();
         for (int i = 0; i < poolSize; i++){
             GameObject particle = new GameObject(); //Create particle
             particle.AddComponent<TextMesh>().text = poem[i].ToString(); //Let particle letter be the next one in the poem
             particle.AddComponent<Rigidbody>();  //Add a rigidbody to the particle
             particle.transform.parent = transform;  //Set the particle as a child of this gameobject, mostly to keep hierarchy cleaner.
             particle.AddComponent<Letter>();    //Add component Letter to keep track of how long it's been alive. This class is written in the same file, check below.
             particle.SetActive(false); //Turn it off for now. 
             letterPool.Add(particle); //Add to pool
         }
 
     }
     
     void Update () {
         timer += Time.deltaTime;
         if(timer > (1/lettersPerSecond)){ //Doing it this way means the maximum letters per secons will be one each frame.
             timer = 0;  //reset timer
             ShowNewParticle();
         }
 
         
         for (int i = lettersAlive.Count - 1; i >= 0; i--){ //Check if any particles should die. Iterating backwards is always best when removing stuff from a list during iteration.
             if(lettersAlive[i].GetComponent<Letter>().timeAlive > aliveTime){ //This isn't very good, GetComponent shouldn't be called in an update. Figure out a better way to do this if it impacts performance. Perhaps with event from Letter?
                 ReturnLetterToPool(i);
             }
         }
     }
 
     /// <summary>
     /// Returns the letter to pool.
     /// </summary>
     /// <param name="index">Index in alive-pool</param>
     private void ReturnLetterToPool(int index){
         lettersAlive[index].SetActive(false); 
         letterPool.Add(lettersAlive[index]);
         lettersAlive.RemoveAt(index);
     }
 
     /// <summary>
     /// Shows the new particle. Select random from letterPool, remove from that pool and add to alive pool
     /// </summary>
     private void ShowNewParticle(){
         if(letterPool.Count == 0){
             Debug.Log("Pool is empty. Chose a longer poem or less particles per second.");
             return;
         }
         int index = Random.Range(0, letterPool.Count-1);
         letterPool[index].transform.position = new Vector3(Random.Range(startPosMin.x, startPosMax.x), Random.Range(startPosMin.y, startPosMax.y), Random.Range(startPosMin.z, startPosMax.z)); //Randomize position
         letterPool[index].SetActive(true);
         letterPool[index].GetComponent<Rigidbody>().velocity = Vector3.zero;
         letterPool[index].GetComponent<Letter>().timeAlive = 0;
         lettersAlive.Add(letterPool[index]);
         letterPool.RemoveAt(index);
     }
 }
 
 /// <summary>
 /// Component added to each particle. Normally this would probably be in a seperate file, but I kept it in the same file for ease of use.
 /// Should be pretty self explanatory
 /// </summary>
 public class Letter : MonoBehaviour{
 
     public float timeAlive = 0;
     private Vector3 rotations = new Vector3(3, 12, 50);
 
     private void OnEnable()
     {
         
         transform.eulerAngles = new Vector3(Random.Range(0, 360), Random.Range(0, 360), Random.Range(0, 360));
     }
 
     private void Update()
     {
         timeAlive += Time.deltaTime;
         transform.Rotate(rotations * Time.deltaTime);
 
     }
 
 }