This is your best bet for finding a legitimate digital copy. The Internet Archive hosts a scanned version of the Brahma Yamala with the Hindi commentary by Siddha Gopinatha and the Sanskrit text edited by Rai Ananta Krishna Svamin.
For those seeking a free PDF version, the original Sanskrit text is in the public domain, as is much of ancient Indian literature. Reputable resources include: brahma yamala tantra pdf free
: Provides a structured English translation/outline of the text, including chapters on the "Extraction of Mantras" and "Secret Signs". This is your best bet for finding a legitimate digital copy
The Brahma Yamala Tantra and The Early Saiva Cult ... - Scribd brahma yamala tantra pdf free