I have a file open in Preview.app. I can easily view the full path for the same by ⌘ + clicking on the file proxy icon (the icon shown besides the filename in Preview's title bar).
How do I copy the full path to clipboard?
Here's how you can copy the full file path.
⌘ + click on the file proxy icon (shown in Preview's title bar). This will reveal the full disk path of the file, shown as hierarchy of directories starting from the root (topmost) directory in the filesystem.
Click on the name of the parent directory (the directory containing the file) of the file. This will open the directory in Finder.app with the file automatically selected.
Performing this action will copy the full pathname of the file in the clipboard. The copied pathname in this example is as shown below:
Using this technique, full pathname of any file can be copied from the Finder.