FolderBrowserDialog is purely ment for Selecting folders ofcourse.
You are looking for OpenFileDialog.
A little example:
OpenFileDialog openFileDialog = new OpenFileDialog();
openFileDialog.Filter = "PDF Files|*.pdf";
if (openFileDialog.ShowDialog() == DialogResult.OK)
string filePath = openFileDialog.FileName;
the filepath string now contains the full path to the file you have selected and you can let your own code loose upon it