The Adyar Library edition is technically under copyright (the latest volumes were published in the late 1990s). However, the Theosophical Society has historically allowed non-commercial, academic circulation of single copies. If you are a student or researcher, you may legally request a digital copy directly from the (they often provide a PDF for a small processing fee). This is the most "verified" route because it comes directly from the source.
The following sources have been verified to provide a reliable English translation of the Sangita Ratnakara in PDF format: sangita ratnakara english translation pdf verified
Often available on digital archives like Internet Archive (Vol 1) and Scribd . 2. C. Kunhan Raja (Adyar Library Series) The Adyar Library edition is technically under copyright
The PDF has been verified by several musicologists and scholars, including: sangita ratnakara english translation pdf verified