Search over different types of documents such as DjVU sometimes without OCR?
I. How to search over different types of documents such as DjVU?
I would convert them first into PDF and then use some well-known OCR-tool such as Adobe Acrobat PRO. You can get it with the Creative Suite to make documents searchable. In order to convert certain type into PDF, I would use Online Converters such as the DjVU Online Converter such as here. The OCR requires a lot of computation power so probably best done with Desktop tools unless cloud-tool or iOS tool found.
II. Online tools? OS X tools? iOS tools? What about files without OCR?
Docspal.com for converting. Desktop tools such as Adobe Acrobat PRO (in Creative Suite) for adding the OCR. PDFPen OS X should also work with the OCR and they have the iOS version as well.
Further information
Looking for Software to Scan or Convert to Searchable and Signable PDF
Make existing PDF searchable ( OCR ) via command line / script and What scriptable OCR Software exists on OSX for a paperless office