You should be able to attach a document to an approval email using flow.
We are now working on the functionality to attach files to Approval requests.
It appears you are trying to format a link through the approval details rather than using the separate Item Link/Item Link Description or Attachments parameters on the approval action in your flow. You can make hyperlinks work in approval details using markdown: Use markdown to format Power Automate approvals - Power Automate | Microsoft Docs but it won't show up as attached to the actionable email in Outlook like the actual attachments would; the same goes for item link parameter as well.
For others in the thread: Attachments don't work with links. The Attachments parameters require the display name of a file and the actual content of the file passed in. To "attach" a link you can use the markdown or the dedicated Item link/Item link Description parameters in the approval action in a flow. We are working on providing support for attaching multiple links and making the UX experience more consistent between "attached" links and file attachments.
A side note: In Dark Mode on OWA I noticed hyperlinks look like plain text so I've raised this issue with the Outlook team.
Hi @TCambs and Stephen (Power Automate),
Is there any work is happening on this feature. That end user can attach a file via approval email in outlook and respond back, and we will be able to manage that out.
@ashwnaidv100 We currently have no plans to add attachments to an approval via a reply/forward email to the initial approval email after the approval has been created. The work done for this feature was originally adding attachments to the approval and having them show up in the approval email that gets sent out. With that said, anything can happen in the future.