Can Pind Daan be done for ancestors who died long ago or are unknown?

Absolutely.

In fact, Gaya Shraddha is especially suited for:

• Forgotten ancestors
• Those whose date of death is unknown
• Souls who died in war, accidents, suicide, or unnatural death Performing Pind Daan in such cases is called “Sarva Pitru Shraddha” and brings peace to all departed souls in the lineage, whether known or unknown. It is a deeply compassionate act of Atma Kalyan (soul elevation) for the entire family tree.