Q: Can I reinstall Snow Leopard over Yosemite?

If you lost the document files, downgrading the computer's OS isn't a solution. Restore them from a backup if possible.

If you lost the ability to open them, copy them and anything else you'd like to keep to another drive, insert the Snow Leopard DVD, restart with the C or Option key held down, use the Disk Utility to erase or repartition the internal drive, and install Snow Leopard onto the blank volume; depending on their type, there may be other solutions.

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Do a backup, preferably 2 separate ones on 2 separate drives.

Revert to a Previous OS X - Yosemite

Revert to Snow Leopard

If you do revert, I'd use Setup Assistant to restore your data. This process takes a while, so do it when you won't need the computer for several hours, based on my experience.