"add an attribute (create a new attribute) for (or within) an existing document"
http://support.ptc.com/WCMS/files/79061/en/WCBusAdminGuide.pdf
Go to Chapter 13, "Type and Attribute Manager" for details. Just wade through it, and play in the Type and Attribute manager to set up your global attributes then add a newly created attribute to an existing or new light type as either a "Document" or "Reference Document."
As for Search, you will be able to use "Advanced Search" for that new attribute once you create your new document IBA on your existing or new document type. Use "Add" to add your instantiable document type with the new attribute, then select that type (only) and then "Add" your new attribute as "Criteria" where you can search on your value with wildcards, like "*my text for search*".
Alternatively, if you have keyword search running properly, which should be the case, then just typing the text for that IBA into the keyword search should yield the result you are looking for. Debugging key word search if you include indexing for content of files, however, if a specialty system admin function that is beyond this particular thread about adding an IBA on a document type and searching for it.
If the manual is too much, and if you have a local Windchill Sales guy, then ask your Sales person to lend you an AE (application engineer) to do a webex and walk you through the basics of some fundamental business admin like this. If PTC thinks you might eventually buy something, then they'll likely spare some AE time to give you some webex knowledge transfer, or maybe come by for a demo and Q&A.
Al