For using a pdf file as a help on client system please use following steps:
First add your pdf file to your setup. You can use any special folder like "ApplicationData" for this purpose.
Then add following lines code on the button of your form to open that file on client machine.
private void button1_Click(object sender, EventArgs e)
string PathOnclientMachine = Environment.GetFolderPath(Environment.SpecialFolder.ApplicationData).ToString();
string helppath = PathOnclientMachine + "\\Help manual.pdf";
MessageBox.Show("Help file has been corrupted";, "Warning", MessageBoxButtons.OK, MessageBoxIcon.Information);
Hope this will help you.