in mac os x, how NOT to bring all the opened window of an application to front

I have a problem with switching between windows in Mac OS X the way I use it.

For example, I have 2 opened windows of PDF files in the background, and on top of them there is another window of excel sheet.

Now when I open a new window of PDF files, I don't want the other 2 windows of PDF that were in the background to come to front.

What can I do?

That's how Mac works.

All windows belong to the same instance of the same app, unlike windows where it's possible to have each window inside a different process.

The simplest workaround is to duplicate the app you need 2 instances of & drag the file you wish to launch to one specific copy of that app.

Alternatively, you might be able to do it in terminal or Applescript/shellscript, but you would need to pass the correct arguments to it on the fly...
do shell script "\"/Applications/myApp.app/Contents/MacOS/myApp\" + arguments [at least file path]

I'm not sure all apps will work with this method, I've only ever tried it with very specific structures with published command line switches.