Use this for a letter you wrote outside Kolleno, so it shows with the customer's other letters. You need the Initiate letters permission.
Only PDF files can be marked as a letter. The Mark as letter option does not show for a Word document or any other type of file. Save the letter as a PDF first, then upload the PDF.
Mark the file
Open the customer, then Files > Uploaded files.
Upload the PDF if it is not there yet.
In the file's row menu, click Mark as letter.
Choose the Letter type and click Mark as letter.
Kolleno keeps the file as it is. It shows on the Letters tab with the source External, and it is eligible for outbound processing at once.