This is one of the know issue for Search in SharePoint if you have CU of October or later installed on your server
Follow below steps to fix the issue.
1. Go to Central Admin --- Manage service application --- Search service application
2. On Search Administration page, Click on Search Schema under Queries and Results.
3. Search for the properties called Title in Managed properties.
4. Once you find the Title property, click on it to edit the property.
5. Scroll Down and Go to Mappings to crawled properties. Select the MetadataExtractorTitle and move it down to last place and Click OK.
6. Perform an Index reset. Click on Index reset under Crawling, select the option to deactivate search alerts and click OK.
7. Once Index resetting is done. Click on Content Sources and start Full Crawl
8. After Full crawl is done, Search result will return filename instead of document title.