At PersistIQ, we currently don't support attachments as they can contribute to deliverability issues. As an alternative, we recommend uploading the document to an online hosting site or storage like Dropbox Basic, Box, or DocSend to name a few.
Here are the steps needed:
Upload the document to an online hosting site.
Upload a PDF image to your template and resize it accordingly.
Get the shareable link of the document from your online hosting account.
Click the image and use the "link" function, then paste the URL link on the URL field.
Don't forget to hit save!
Aside from that, you may also just hyperlink the document URL to a text if preferred. There are also available free PDF thumbnail generator online if that helps. :)