You can pass Adobe commands as query string parameters to the web site link. E.g
http://myapp/mypdf.pdf?#zoom=100%&view=fit&navpanes=0&toolbar=0
will change zoom to 100 %, view to fit , no navigation panes and no tool bar.
For more look
this [
^] link.
EDITED
Well, accept the parameters from user input controls such as textbox or combobox. i.e
private void button1_Click(object sender, System.EventArgs e)
{
string zoomFactor = int.Parse(TextBox1.Text);
WebBrowser1.Navigate("http://mpapp/mypdf/roles.pdf?#zoom=" + zoomFactor + "%&navpanes=0&toolbar=0");
}