Cannot delete file becouse it is used by another process.

Question

Hi all,
I dont understand my problem. In DocumentPage I upload pdf file ->I show it in dataGrid -> after select this dtg row ti create pdf file in some folder and at the same time it show it in webBrowser. After exit applicatin I am trying to delete this file but it shows me en arror mentioned in question title.

my exit function:

App.xaml

<Application ...
    Exit="App_Exit"
>
...
</Application>

App.xaml.cs

public partial class App : Application
    {
        void App_Exit(object sender, ExitEventArgs e)
        {
            DocumentPage z = new DocumentPage();
            z.clear();
            if (Directory.Exists("temp"))
            {
                Directory.Delete("temp", true);
            }
     }
}

... here it shows me that eror message

1. In same page (DocumentPage) I have button with function ( clear() ) : pdfDokument.Navigate("about:blank");

after click on that button ... application exit and sucessfull delete folder temp with all files inside

SO I dont understand why it is ok with click on that button and when I use this function in app_exit ... it doesnt work