If you are a serious practitioner, a free PDF may be difficult to read due to poor scanning or lack of commentary. The text is highly technical and cryptic. It is highly recommended to invest in a printed edition or a paid digital edition that includes a commentary by a modern master. The most popular and accessible version is "Swarodaya Vidya" (or similar titles) by Swami Satyananda Saraswati of the Bihar School of Yoga , which explains the esoteric concepts in modern scientific language.