Adobe Acrobat 7 Professional - Manual (Page 567)

Adding custom properties
Use the Custom Properties option to allow index users to search within the custom
properties of PDF documents. For example, when you create an index, you can add the
custom property Document_Name and assign a property type of string. A user searching
the index can then select the index and search within the custom property by selecting
Document_Name from the Use These Additional Criteria menu. (See
Creating document
properties .)
To add custom properties:
1. Choose Advanced > Catalog and then click New Index or Edit Index.
2. Click Options, and then click Custom Properties.
3. In the Custom Property text box, enter the custom property name.
4. Select a property type from the Type menu, and then click Add.