private void OpenFilesCommand_Execute()
        {
            var dialog = new OpenFileDialog
            {
                Multiselect = true,
                Filter      = "PDF files (*.pdf) | *.pdf"
            };

            if (dialog.ShowDialog() == true)
            {
                PdfFiles.AddRange(dialog.FileNames);
            }

            ExtractImagesCommand?.RaiseCanExecuteChanged();
            ClearFilesCommand?.RaiseCanExecuteChanged();
        }