It's possible to move files to trash via Finder with ⌘⌫. But if i have an opened file (e.g. pdf or txt), is there any way to directly move it to trash without first closing it and finding it in Finder? Any shortcut?
The shortest way: Right-click the file icon in the title bar of the document window. It will display a drop-down menu of the file's location path. When you choose the folder icon that is directly under the file icon, a Finder window will open with the file already selected (highlighted), and you can directly press ⌘⌫ to trash it.
It depends on the app, which has to implement this, not a general OS X keystroke.
In Preview: ⌘⌫
TextEdit seems to have nothing.