It would be useful to “Copy complete file name” (including full path). This would be especially useful for emailing a recently created file. This is an option in another good viewer PDF-XChange Viewer.
It is not documented but you can go to file properties [Ctrl +D] and hit [Ctrl +C] to copy the file details to the clipboard the first line is the full filename (prefixed by the word File:) and that line is what is shown by most one line dialog boxes so is sometimes easy to paste into say explorer OR should work as subject line?. I agree it not ideal as you need to remove the word File: or the additional lines if you paste in an email.
A quicker alternative for just the location and short filename is to press F2, the filename is highlighted so easy to copy and the folder address is usually easy to copy from address bar.
Both are admittedly work-arounds and the ability to show the filename with one click has been requested before (usually for use with tabs)