If you cannot find a safe download link immediately, here is the core text of the 21 precepts (Standard Translation) that you can save for your own records:
Many university philosophy and East Asian studies departments host public-domain texts for course use. Because the Dokkōdō was written in 1645, the original Japanese is copyright-free. However, translations have varying copyrights. To be safe: miyamoto musashi dokkodo pdf verified download
: The University of Buenos Aires (UBA) provides a comprehensive PDF that includes the 21 precepts along with modern interpretations for leadership and personal growth. If you cannot find a safe download link