Plato can be used to edit Fortran files without adding them to a project. Such files will exhibit syntax colouring but (within Plato) they can only be compiled and built as stand alone files. In order to open a file, click on the Open File button on the main toolbar. Now using the standard Open dialog box, select Fortran files of type Free format or Fixed format from the drop down list.
Now navigate through the folders and select the file that you want to open.
Alternatively you can open a Fortran file from Explorer either by double clicking on the file or by dragging the file and dropping it on to Plato.
The double click event requires Plato to be linked to Explorer for opening Fortran files. To provide this link you can select "Register Fortran file types for Explorer" from the Plato Options dialog (on the main Tools menu) or you can use Explorer to change the Folder options (using the Tools menu item in the main Explorer menu).