Andrei Zhitkov

Defining your default canonical arbitrary is most useful for those unfortunate souls who have their site URLs in punycode. This plugin allows setting a default custom URL to be used as canonical. It is designed to work with Yoast SEO only. The individual settings are still available and can be edited as before. Since it is dependent on presence of Yoast SEO, this plugin will not activate without it. 
The plugin does not add its own settings page, It uses WordPress Settings -> General. Look for it at the bottom of that page.
<h3>Technical Details</h3>
Supported versions: 
WordPress 4.x 
WordPress 5.x 
WordPress 6.x 
Yoast SEO 6.x-20.x

Custom canonical URL for Yoast SEO

This plugin allows setting a default custom URL to be used as canonical. It is designed to work with Yoast SEO only.

Jake Bohall

hivedigital

Ionut Boicu

This plugin was developed to allow canonical tags as response headers to be created via htaccess for attachment files such as pdfs and docs.
The plugin:
<ul>
<li>Generates an htaccess file to manage the wp-content/uploads directory where these files are being uploaded</li>
<li>Updates the htaccess file with canonical tags which are delivered via a header when the attachment is called</li>
<li>Creates a dashboard where you can easily filter and find attachments without canonicals or that are not attached to a post.</li>
<li>Creates a dashboard where you can easily attach a file to its parent post (one at a time or in bulk) or manually add/remove a canonical tag for a file</li>
<li>Allows you to add a canonical url from the media file in the library or when you upload</li>
<li>Creates a dashboard to edit the .htacess file directly for the uploads directory in case there are additional files in the directory that were uploaded outside of wordpress (e.g. during a site migration)</li>
</ul>
The challenge: Analytics software such as Google Analytics, cannot track a user that enters a website via a pdf file in Google search results. Files cannot have a standard canonical tag added in the <head> and instead the canonical tag has to be added via an http header when the file is requested from the server. In order to accomplish this, we need to have an htaccess file listening for a request to that file, and then sending the header.
Expected outcome: PDF files and other attachments will be replaced in search results with the post or page counterparts, and we will be able to measure the traffic to those pages and provide calls to action for downloading the file, submitting contact forms, etc. Note: Canonical tags do not force a search engine to index one url vs. another, but act as a guideline/request to the search engine. As long as the attachment content is similar to the canonical URL, you should have no problem encouraging the search engines to choose the correct version.
<h3>Future Features and Development</h3>
Based on the feedback, we hope to continue developing additional features to support user requests. Current future feature ideas include:
<ul>
<li>Automatic canonicalization of non-image files to attached post</li>
<li>Integration with non-apache servers</li>
<li>Canonical builder for non-wordpress media files</li>
<li>Option to bulk delete canonical associations and delete htaccess on uninstall</li>
</ul>

Canonical Attachments

The plugin is easy to install and the admin interface makes it super simple to quickly add the canonical tag to pdf files.

As one of the developers of the plugin, I am biased, but ... I can honestly say that it accomplishes everything that I need.   



Works exactly as expected!

Get more information from analytics and higher rankings by utilizing canonical http headers in htaccess for non image media attachments like pdfs.

Custom canonical URL for Yoast SEO vs Canonical Attachments