Hi,
Try this:
Response.Write("<script> window.open(ViewDocuments.aspx?id='"+ e.CommandArgument.ToString()+"')</script>");
But this is not best practice. Try this for best practice:
string script = "window.open('popupPage.aspx'"+e.CommandArgument.ToString()+"'', 'myPopup')";
ScriptManager.RegisterStartupScript(Page, Page.GetType(), "someId", script, true);
All the best.
--Amit