[colicoid]

If it is an actionable item as well as a reference item I would do it like this:

Project support materials -> Learn GTD and OF (folders in file system)
Learn GTD and OF (Project in OF)
Study the Get started with OmniFocus document (action in OF)
@read (context in OF)

The pdf called "Getting started with OF" I put inside the folder in the file system. I then create a link between the folder and the OF project so I can access the folder from within OF. I create a new action in the project and give it the context of @read. If I feel particularly anal about it I might establish a link between the actual action and the file as well.

My @read context almost never becomes swamped with things because I put less interesting projects on hold so my @read context is filled with juicy stuff that I want to read (or, sadly, boring important stuff).