Well, I tried logging the variable with the URL to the history list before I did anything, but shows nothing when I look at the history. Maybe I'm using the wrong metadata?

The build string just puts several workflow variables together.

The first attachment is an example from the "Insert Reference" button within the Send Email action.

It just emails a link to the current item and correctly handles cases where a space is in the URL.

Perhaps this could be a starting point?

The second attachment is from the "Set Workflow Variable" action that's assigning the current item URL to a text variable.

If you want to send an email for every item that satisfies a certain criteria then you maybe use a list workflow with an insert and/or update trigger filter to match your "certain documents" requirement. Otherwise with large lists you can encounter Microsoft's processing governor limitations that'll pause your workflow for 24 hours.

I'm using a site workflow rather than a library one, but I think the set variable could work! I'm trying to build the URL dynamically using the ID of the document, but am having issues. All the information I am finding is for list items rather than documents.

This is a site workflow and I am now using a set variable action. I was able to dynamically build the hyperlink. When I log it to the history list, I can see that it is correct and indeed a link. However, when I put that into a build string and email it out, I get extra and </div>. I'm also unconvinced that removing these extra characters will result in a clickable link in the email, though I could be wrong. What I get is the following:

https://URL/sites/Site/Library/FileName.FileType</div>

Does anyone know how I can fix this?