Yes, although it can depend on the types of files involved and/or if IFilters for those types are installed, if the text isn't in them in plain text.
It's generally the same as what Windows Search in terms of contents. (And you can use Windows Search itself within Opus, if you wish, although Opus also has its own Find Files panel that doesn't use it.)
If you need more detail on anything or have any other questions, please start new threads for them: