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 copyrighted book is not legally available, here are the best alternatives: himnario adventista antiguo del 1 al 527 descargar
For many Spanish-speaking Seventh-day Adventists, the Himnario Adventista Antiguo (the "Old Adventist Hymnal") is more than just a book of songs. It is a vessel of memories—echoes of camp meetings, Sabbath mornings in small country churches, and the powerful harmonies of hymns like "Perdido Andaba" and "Oh, Yo No Sé." It is a vessel of memories—echoes of camp
A Treasured Legacy: Downloading the Old Adventist Hymnal (Himnario Adventista Antiguo – Hymns 1 to 527) Sabbath mornings in small country churches
The Internet Archive (archive.org) hosts many older Adventist songbooks from the 1920s-1940s that contain the same classic hymns (often with different numbering). Search for "Himnario Adventista 1940" or "Himnario Evangelico." These are fully legal to download as PDFs.