Unable to delete PDF file

Question

I open a PDF file in brower which is downloaded from server.
I want to delete the PDF file when I click the "X" button of browser, so I use 【file.Delete()】 of C# program ("file" is FileInfo() type) to delete the PDF file. But it not always works, sometimes the PDF file cannot be deleted and the browser has no corresponding.

If I close the browser, and open the folder where the PDF is locating, delete it in windows menu, then it will load all the time(the pic below) until restart the PC.

I want to know what happens to the PDF file?How can I delete it with C#?

Answer 1

Next, if I double click the PDF files, Adobe Reader will be open and show the message below. Yes, the file is deleted.