I am using .UnifiedSearch to search in pages and .pdf/.docx-files and I am pretty satisfied with how it works all around, except that it is very poor at searching in files.
When I use .UnifiedSearch, only words that are used very frequently in the document will return the document as a search hit.
When I use .Search the search engine returns matches for any word in the document.
Why is there a difference? Can I configure my index so that UnifiedSearch returns better matches for documents aswell?
I have no anwser for you question but we have hade the same experience. Not sure if you can change anything in your index. I would sugest you start a support ticket and write the answer from EPiServer in this thread :) Since I am curious as well.
If you need this functionallity badly and have a problem would it be possible to create two searches? One for files and one for other content as well.
I would recommend trying the AttachmentHelper package provided by EPiServer to increase relevancy of searches in documents. There is some information under "Improving search relevancy for attachments" on http://world.episerver.com/documentation/Items/Developers-Guide/EPiServer-Find/10/DotNET-Client-API/File-attachments/.
This will help when indexing since the content will be using analyzers from all supported languages (as when indexing normal content) and it will also make the actual indexed text show in the browser in the Find Admin UI instead of the nasty Base64 encoded string you would see otherwise.