How to Decrypt a SHA256 Base64 Hash from PDF in C#

Question

I Have a hash that I've created from a PDF file with the function below:

using System.Security.Cryptography;

private readonly SHA256 fileToSha256 = SHA256.Create();

public string FileToBase64SHA256(string fileName)
{
   using(FileStream stream = File.OpenRead(fileName))
   {
      byte[] streamToSHA = fileToSha256.ComputeHash(stream);                
      return Convert.ToBase64String(streamToSHA);
   }
}

How could I Decrypt the result hash above and save it in a PDF file again? I am trying something like this, but the file saved is corrupted, with strange characters:

public string HashToFile(string hashedFile, string fileName)
{
    BinaryWriter Writer = null;        
    byte[] data;

    try
    {
        Writer = new BinaryWriter(File.OpenWrite(fileName));

        data = Convert.FromBase64String(hashedFile);              
        Writer.Write(data);
        Writer.Flush();
        Writer.Close();
        
        return "Saved Successfully";
    }
    catch
    {
        return "Error converting to File";
    }
}