If, for any queries, you cannot see the files in the search results (and in some cases, even publications (posts) are not displayed in the search results too), then most likely the plugin conflicts with another plugin or with the settings of the theme itself, or your files simply cannot be indexed.
To detect the exact reason and solve the problem, please do the following:
Make sure that the files you are trying to find using the search widget on the frontend of your website are stored in the WordPress Media Library. Without special settings, the WPFTS Pro plugin should only index files that are saved in the WP Media Library.
If your files are indeed in the WP Media Library, then you need to check if the text content is extracted from the files correctly.
Select one of the files and go to its Edit Attachment page.
On this page, find the "WPFTS Extracted Content" box. Make sure this box contains the text that was inside your file. If this is not the case, you will probably see an error message above the window explaining why the specific content was not retrieved. Possible reasons are: wrong license key (or not active license) or the file is not extractible.
Do you see the extracted text and still can not find files by content? Then go further.
Click the Plugins item in the main WP-Admin menu and look for plugins that might affect search functionality. You've probably tried some other search-related plugins before installing WPFTS Pro but haven't yet deactivated them. Now you need to disable them all (at least while you are trying to work with WPFTS). Different search plugins always tend to conflict with each other. Then try to search again.
If that doesn't help, then go to your theme's settings and look for search-related options. If your theme allows you to disable those search options, do so.
If the above did not help, check with your developer if he uses WP hooks typical for search and filtering, such as pre_get_posts and others, in his custom code. If so, you will need to ask him to disable these pieces of code (probably temporarily).
Still not working? Feel free to post a new question on the forum in the "Bugs and Fixes" section.