using System;

public class Program

{

    public static void Main()

    {

        var fileName = "PBS_123_456_7890_9.pdf";

        var fileNameParts = fileName.Split('.');

        var docIdParts = fileNameParts[0].LastIndexOf("_");

        var docId = fileNameParts[0].Substring(fileNameParts[0].LastIndexOf("_") +1).Length == 1 ? fileNameParts[0].Substring(0, docIdParts) : fileNameParts[0];

        Console.WriteLine(string.Format("Hello World: {0}", docId));

    }

}