Michel Kusters

This plugin adds a download link below every image in your WordPress galleries.
<h3>Gutenberg</h3>
This plugin currently only affects the old-school <a href="https://codex.wordpress.org/Gallery_Shortcode" rel="nofollow ugc">gallery shortcode</a>. The gallery block, introduced with Gutenberg in WordPress 5.0, is not affected.

WP Gallery Download Links

This plugin adds a download link below every image in your WordPress galleries.

HuangYe WuDeng

This plugin generates PDF documents for visitors when they click the “Download as PDF” button below the post. Very useful if you plan to share your posts in PDF format. 
Note: You can replace the logo file logo.pngunder wp-content/plugins/down-as-pdf/images/ with your own. 
注意：请将wp-content/plugins/down-as-pdf/images/目录下面的logo.png文件替换成你自己网站的logo
<ul>
<li>中文介绍请到<a href="http://ihacklog.com/?p=3771" title="插件主页" rel="nofollow ugc">插件主页</a></li>
</ul>

Hacklog Down As PDF

This plugin generates PDF documents for visitors when they click the "Download as PDF" button below the post.

WP Gallery Download Links vs Hacklog Down As PDF