I received several questions about indexing of SharePoint content lately. Specifically does SharePoint index the entire document?
First off if FAST is being utilized, yes, the entire document will be indexed.
Second if you are using the out of the box SharePoint search by default it configured to index the content of items that are under 16MB or under. Metadata associated to these documents will still be indexed (so if the big files are videos they will still be searchable).
The max file size for indexing can be upped up to the max file size limit; which is 2GB. Before upping the size, it is recommended that you do some testing and understand any performance ramifications relative to your environment and data. Here are some good references on the topic.