Attaching files to a project
Each of the three main inspector windows (Project, Task, and Resource) has an Attachments inspector. You can use this to keep references to files that are relevant to your project or to particular items in it.
Click the item to select it.
In the Task Attachments inspector or the Resource Attachments inspector, click the plus button below the attachment list, then choose a file in the dialog that appears. If you are showing the Attachments column in the outline, you could also click the paper clip icon and choose Attach File from the pop-up menu, then choose a file. Or, easiest of all, just drag a file into the attachments list.
The file appears in the inspector's attachments list.
To attach a file to the project itself:
In the Project Attachments inspector, click the plus button below the attachment list, then choose a file in the dialog that appears. Or, just drag a file into the attachments list.
The file appears in the inspector's attachments list.
You can open an attached file by selecting it in the appropriate Attachments inspector and then clicking the Open button, or by choosing it from the pop-up menu in an item's Attachment cell in the outline.
Every file attachment is a reference to the file elsewhere on your disk. If you move the files around, the references try to keep track of the new location. But if you are going to send a project to someone else, or otherwise move your project to another computer, the other files won't automatically come along for the ride. In that case you should gather all of the attached files into one folder along with the OmniPlan file itself, and send the whole folder.
You can show attachments as a column in the task and resource outlines by choosing View Options from the View menu.