Since a direct "free download" of the full copyrighted book is not legally available, here are the best alternatives:
Himnario Adventista: para uso en el culto divino | Hymnary.org himnario adventista antiguo del 1 al 527 descargar
Some historical societies and private collectors have scanned the old hymnal for research purposes. A Google search for "Himnario Antiguo 527 PDF" might yield results, but verify if the uploader has permission. Since a direct "free download" of the full