Ooypunk

Once upon a time there was a website in the auld style, with imagery in an images directory. But then it was said that 
the website should be built anew, but in WordPress, and the images were transported by FTP to the new structure. But 
the new pages and posts needed images, but the images were not in the Library. 
This plugin scans the upload directory for images that are not yet in the Library, and provides links for each one to 
be included.

CIT Media Sync

Plugin to add files to the Media Library that were previously uploaded by FTP.

Pranav Pathak

The Agreement plugin helps to create pop-up licence agreement or term to use before download any thing from post or page. It will give freedom to change text in pop-up. After activation of plugin in post/page add file from “Add Media”, then change editor type from Visual to text and give id to that media file. Id will be like this id=”agreement”. Your link will be like ‘id=”agreement” href=”http://yourdomain.com/wp-content/uploads/2016/01/pdf.pdf”‘ or you can use custom HTML block too. Now save post/page.
Example
{a id=”agreement” href=”http://yourdomain.com/wp-content/uploads/2016/01/pdf.pdf”} file name {a}
This plugin works for multiple item in single page.
After activation there is a sub menu in Settings called Agreement. Click on this and there is a option to change license agreement text called “Enter your License Agreement setting text below:”. You can also insert html tag like ul,li, br, hr from agreement option in settings.
<h4>Requirements</h4>
<ul>
<li>WordPress 3.5.1 or greater</li>
<li>PHP 5.2.6 or greater (5.3 recommended)</li>
<li>MySQL 4.1.2 or greater (5.x recommended)</li>
</ul>

Agreement

This is the best and most simple plugin I have found for this purpose. 

Great plugin

The Agreement plugin helps to create pop-up licence agreement or term to use before download any thing from post or page.

CIT Media Sync vs Agreement