How to get filename of currently open document application.

Hi Friends!!
I am beginer to C#.
I am developing Password Pro application in that i want to automatically detect the password of currently opened file application from access database file .
Generally we have secured our data in password protected file say docx, xls, pdf etc.
I want to make application such that when i cliked on particular password protected file the password gets detected from database and simply shown to user.

I am capture the title of currently opened dialog box i.e. "Enter the password" window which is child window of any application like docx, pdf etc.
But I want to need the title of file i.e. file name of currently opened application.
for example demo.pdf, op.docx etc.
when i clicked on say demo.pdf i want to need this demo.pdf(file name)

Please anybody help me and give me code that are need for getting this currently opened document application filename.