I worked at an MSP for years and had clients who would send it back and forth that way to each other. The most common were medical practices, but banking wasn't far behind.

Fax was considered secure because you could lock up the fax machine physically.

My favorite setup was a medical practice that dealt with stomach issues, including colon cancer, and their neighbor in the same building who was a pathology lab. They both had the PDF to fax and fax to PDF servers set up to send back and forth when someone could just walk 50 feet and hand over paperwork.