ChrisHurst

MyWebsiteAdvisor

This plugin allows you to watermark your previously uploaded images using a PNG image file with transparency as well as text signature. 
The user friendly settings page allows for control over the appearance of your watermark. 
You can set your watermarks to always be a specific percentage of the target image. 
This plugin will add the watermarks to ALL of the images in your WordPress uploads directory.
Please BACK UP all of your uploads via FTP before using this plugin!
<a href="http://mywebsiteadvisor.com/plugins/bulk-watermark/" rel="nofollow ugc">Upgrade to Bulk Watermark Ultra</a> for advanced 
watermark features including:
<ul>
<li>Fully Adjustable Text and Image Watermark Positions</li>
<li>Adjustable JPEG Image Output Quality</li>
<li>Highest Quality Watermarks using Image Re-sampling rather than Re-sizing</li>
<li>Lifetime Priority Support and Update License</li>
</ul>
Check out the <a href="http://www.youtube.com/watch?v=XkFXBjfzw2I&hd=1" rel="nofollow ugc">Bulk Watermark Plugin for WordPress Video Tutorial</a>:
<iframe loading="lazy" title="Bulk Watermark for WordPress Tutorial" width="750" height="422" src="https://www.youtube.com/embed/XkFXBjfzw2I?feature=oembed" frameborder="0" allow="accelerometer; autoplay; clipboard-write; encrypted-media; gyroscope; picture-in-picture; web-share" referrerpolicy="strict-origin-when-cross-origin" allowfullscreen></iframe>
Developer Website: http://MyWebsiteAdvisor.com/
Plugin Support: http://MyWebsiteAdvisor.com/support/
Plugin Page: http://mywebsiteadvisor.com/plugins/bulk-watermark/
Compare Watermark Plugins: http://MyWebsiteAdvisor.com/tools/wordpress-plugins/watermark-plugins-for-wordpress/
Video Tutorial: http://mywebsiteadvisor.com/learning/video-tutorials/bulk-watermark-tutorial/
Requirements:
<ul>
<li>PHP v5.0+</li>
<li>WordPress v3.3+</li>
<li>GD extension for PHP</li>
<li>FreeType extension for PHP</li>
</ul>
To-do:

Bulk Watermark

Adds an image and/or text watermark to all uploaded images, using PNG images with transparency.

iLovePDF

Compress your PDF files and Stamp Images or text into PDF files. This is the Official iLovePDF plugin for WordPress. You can optimize all your PDF and stamp them automatically as you do in ilovepdf.com.
<h4>How it works</h4>
The plugin has two modes of work, automatic or manual. 
Compress PDF: Every time a PDF file is uploaded to your Media Library, is compressed by our iLovePDF API and saved in your WordPress site already optimized, saving you disk space. This feature can be disabled.
It can also compress all PDF already in your Media Library at once or compress PDF files one by one. The compression ratio depends on the PDF but on average you can save up to 50% of disk space occupied by PDF files without loosing quality.
Watermark PDF: Every time a PDF file is uploaded to your Media Library, is stamped by our iLovePDF API and saved in your WordPress. This feature can be disabled. 
It can also stamp images or text in PDF already in your Media Library at once or stamp PDF files one by one.
<h4>Bulk actions</h4>
All tools can be done for an individual files or applied to all your existing files.
<h4>Automatic process</h4>
All tools can be applied automatically when a PDF file is upload, so you don’t need to apply any manual anymore.
<h3>Configure your iLoveAPI Developer Account</h3>
Install this plugin and go to <code>Settings -> iLovePDF</code> to create your iLoveAPI Developer account (if you already registered previously on <a href="https://iloveapi.com" rel="nofollow ugc">iLoveAPI Developer</a> you can login directly) and choose your Project, but by default it will be selected Default Project. A project is what defines which API keys will be used. If you need it, you can manage your projects in your iLoveAPI developer account. With your iLoveAPI Free account you get 2500 free credits to process per month. If you need more you can purchase them
By going to <code>Settings -> iLovePDF -> Compress PDF</code> you can set the compression level you prefer to optimize your PDF files. We strongly recommend you to use ‘Recommended Compression’ which offers the perfect balance between compression and quality. In this section you can disable the auto compression of new uploaded PDF files in your Media Library.
By going to <code>Settings -> iLovePDF -> Watermark PDF</code> you can set many options on what to stamp and how into your PDF files. In this section you can disable the auto stamping of new uploaded PDF files in your Media Library.
To Compress or Stamp PDF files that are already in your Media Library go to <code>Media -> iLovePDF -> Compress PDF or Media -> iLovePDF -> Watermark</code>

PDF Compressor & Watermark – iLovePDF

works well

Best PDF compress plugin

Optimize and protect your PDFs with automatic compression and watermarking. Save space and secure your documents effortlessly.

Bulk Watermark vs PDF Compressor & Watermark – iLovePDF