Attaching files and notes to actions and projects
You can use the notes area of an action or a project to store extra information or materials about the item.
When you have an action or a project selected, click the notes icon 
on the far right side of the row (or press Command-apostrophe). You can type, paste, or drop plain text, rich text, links, images, or other files in this area. Click the notes icon again to hide the notes and attachments.
on the far right side of the row (or press Command-apostrophe). You can type, paste, or drop plain text, rich text, links, images, or other files in this area. Click the notes icon again to hide the notes and attachments. Another way to attach a file to an item is to select the item and then choose Attach File from the Edit menu.
By default, OmniFocus creates links to the files you attach, rather than adding the files to your OmniFocus database. This keeps your database from growing needlessly (which is especially important if you are syncing your data), but the link will break if you delete the file or put it somewhere OmniFocus can’t find it. To make OmniFocus copy a file into its database for safekeeping, hold the Option key as you drop the file, or select Embed the file in the document in the Attach File dialog.
An item with notes or attachments shows its notes icon all the time; others only show it when you select them or put the mouse pointer over them.