Email attachments are a backbone of many companies and for many people can be very convenient methods of transferring files. It isn't perfect, but if you need to send a file to someone and it's not ridiculously large, it can work in a pinch.
Gmail is trying to make email attachments easier, with Google's famous searching capabilities expanding further across your inbox. It was always possible to search for an attachment if you knew the file name, but searching inside a document for some text or a phrase is new.
That's exactly what Google is bringing to the table, indexing documents so that they can be searched from within an inbox. Not all files are indexed yet, but if you've got a PDF or a document and want to track it down, it's now a possibility.
It seems that this is rolling out progressively, with some accounts getting access before others. No doubt there will be some questioning of how Google is indexing files, but for the average user this should be a nifty little feature to hunt out documents they could have previously assumed lost.
One comment suggests that this is rolling out to Latin characters before any others, explaining that they have been unable to use the search with other alphabets, such as Chinese or Korean.
Source: Googlesystem Blog