You can attach a PDF version of a Snailer template to any receipt that you send out from Kindful.
You need to make sure you have that letter you want to send as a PDF setup as a Snailer Template that is set as a Transaction Receipt (so it can be emailed).
Then, when you head to Settings > Fundraising Settings > Transaction Settings, there is a checkbox you can select in the Default Transaction Receipt Emails section to enable this PDF option.
Once you enable this, you will be presented with a dropdown to include the attachment (template) you would like to have included! Check out the picture below of what this looks like.