Declare Auto Function FindWindow Lib "USER32.DLL" ( _
        ByVal lpClassName As String, _
        ByVal lpWindowName As String) As IntPtr
 
    ' Activate an application window.
    Declare Auto Function SetForegroundWindow Lib "USER32.DLL" _
        (ByVal hWnd As IntPtr) As Boolean
 
    ' Send a series of key presses to the Calculator application.
    Private Sub button1_Click(ByVal sender As Object, _
        ByVal e As EventArgs) Handles button1.Click
        System.Diagnostics.Process.Start("C:\Preiskalk.pdf")
        ' Get a handle to the Calculator application. The window class
        ' and window name were obtained using the Spy++ tool.
        Dim calculatorHandle As IntPtr = FindWindow("Adobe Reader", NULL)
 
        ' Verify that Calclator is a running process.
        If calculatorHandle = IntPtr.Zero Then
            MsgBox("Adobe Reader is not running.")
            Return
        End If
 
        ' Make Calculator the foreground application and send it 
        ' a set of calculations.
        SetForegroundWindow(calculatorHandle)
        SendKeys.SendWait("ASDF")
        SendKeys.SendWait("{ENTER}")

Dim calculatorHandle As IntPtr = FindWindow("Adobe Reader", NULL)