you can use System.Diagnostics.Process.Start as well as WIN32 ShellExecute function by means of interop, for opening PDF files using the default viewer:
System.Diagnostics.Process.Start("SOMEAPP.EXE","Path/SomeFile.Ext");
[System.Runtime.InteropServices.DllImport("shell32. dll")]
private static extern long ShellExecute(Int32 hWnd, string lpOperation,
string lpFile, string lpParameters,
string lpDirectory, long nShowCmd);
Another approach is to place a WebBrowser Control into your Form and then use the Navigate method for opening the PDF file:
ThewebBrowserControl.Navigate(@"c:\the_file.pdf");