When you open a source file, its name is added to the Window menu. You cannot use the Open command on the File menu to open another copy of an open source file.
To open a file
From the File menu, choose Open.
Select the drive and directory where the file is stored.
Specify the types of files to display in the Files of type box. Files with the chosen extension are displayed in the list box. The Files of type box initially lists commonly used file extensions.
Select a filename, then click Open.
You can also open a file by double-clicking the file icon in the Target tab of the Workspace window.
The names of the four most recently opened files are displayed at the end of the File menu. To open one of these files, choose its name from the menu.