SharePoint is only able to set permissions for discrete objects (lists, libraries, sites, documents, etc.) and is unable to segment an individual file.
We do two different things here at work to deal with this type of need. We either create a library for a given document then have individual files for each section (this is also useful for collaborating on huge documents even if you don't need to restrict access per section) or we create individual libraries for the sections. The latter is a better way to go for security because it reduces the risk that someone will create a doc and not set permissions.
FWIW: I did a quick check to see if Word would allow me to DRM sections of a document, the answer is no.
Cheers,
Reeves