The Sitecore.ContentSearch.BuiltinFields.Content
field in the index only contains content from binary files in the media library. If you look at the config it references Sitecore.ContentSearch.ComputedFields.MediaItemContentExtractor
.
To search across all fields you would need to add a custom IComputedIndexField
to the <fields hint="raw:AddComputedIndexField">
which aggregated all the fields you want to search on, or just include all the fields you want to search on in your linq query.