Is it possible to find a torrent in DHT network without knowing its hash?

However, I have concern if someone can find these torrents using DHT. Is it really possible (without knowing hash)?

yes

And how do search engines like btdigg enumerate DHT torrents? Is it possible to enumerate (nearly) all of torrents registered inside DHT network?

The DHT as originally specified is not enumerable per se, but an extension (BEP 51) aims to change that. But even without that each lookup for a torrent visits many nodes in the process, so it is possible for nodes to passively observe traffic and gather infohashes over time. It's not deterministic, but a large fraction of infohashes can be gathered that way.

For the moment your best approach should be to encrypt the files before torrenting them. E.g. 7zip provides fairly strong encryption.