Help file/Application path - Urgent Help

I created an installer to my application and I created sub folder (help) under the application folder and then
added Help pdf file. But how to get my application path for the help file then?

I mean, the help file must be in the application folder after installing it. But what code shall I use to get it
so that I can code it in my application and build it.

This way, when the user runs the application after installing it, he will be able to display
the help file if he clicked on the help menu

I tried this but didn't work. What did I miss and what do I need. Can you write the code?

string exepath = Application.StartupPath + "\\HelpFile.pdf";
Process.Start(exepath