This is a well identified issue which was fixed with the Oct 2013 CU update to SP 2013.
This happens because the managed property ‘Title’ is mapped to crawled property ‘MetadataExtractorTitle’ by default as first priority and there was no way to change the order of this crawled property unless October 2013 Cumulative Update was applied.
However, this issue has been addressed in October 2013 Cumulative Update. So, in order to fix this issue without a display template workaround, download and install October 2013 Cumulative Update for SharePoint 2013 on all Web Front End Servers, reboot the machines and run the Products Configuration Wizard on all servers.
Once the upgrade is complete, go to Central Administration > Search Service Application > Search Schema and search for managed property ‘Title.’ Now, inside the mappings of the managed property, you should be able to see the following:
'MetadataExtractorTitle' -- crawled property becomes visible after applying October 2013 CU
Move the mapped crawled property ‘MetadataExtractorTitle’ down to the last position and save the managed property. Then reset the index (optional), perform a full crawl against the content sources and refresh any cache associated with Site Collection. Once the full crawl is complete, you should see the correct titles for documents in search results,
Link to furthern reading here: http://consultant.tamjid.com/425/fixed-search-results-not-showing-document-titles/