Hi,

i solved problem which many people have here with AWS DocumentDB. Its simply because AWS DocumentDB is not public accessible out of VPC. In default its configured its accessible only from same VPC.
Here is a official tutorial how can you connect from outside of VPC (like from localhost) - trought SSH tunnel (like when you using AWS Aurora) https://docs.aws.amazon.com/documentdb/latest/developerguide/connect-from-outside-a-vpc.html

Everything works for me now :)