Save PDF via Background Task

Thanks for Reply

I have implement your suggestion to my code but that gives me error.

Sub New(ByVal _corelApp As Corel.Interop.VGCore.Application)
    InitializeComponent()
    Me.corelApp = _corelApp
    If bgTask Is Nothing Then
    bgTask = New BGTask(corelApp)
    bgTask.RunningChange += AddressOf BgTask_RunningChange

End If
End Sub

Error  'Public Event RunningChange()' is an event, and cannot be called directly. Use a 'RaiseEvent' statement to raise an event.

If I remove line "bgTask.RunningChange += AddressOf BgTask_RunningChange"  then error was cleared but file is not exported in background task.

Does't work. It's block UI and hangs in looping for all pages.

I don't have problem in publish to pdf, your publish logic is in "RunTask" method ? 

Public Sub RunTask() Implements ICUIBackgroundTask.RunTask
    Running = True
    SavePDF
End Sub

Protected Sub SavePDF()
    Try
        app.ActiveDocument.PublishToPDF(filename)

    Catch ex As Exception
        app.FrameWork.ShowMessageBox(ex.Message)
    End Try
    Return
End Sub