I followed all steps mentioned on this page. https://docs.aws.amazon.com/dms/latest/userguide/target.docdb.html
The problem is that the "import certificate" button is disabled and I can not upload the certificate.
You can download the public key for Amazon DocumentDB as the rds-combined-ca-bundle.pem file from an AWS-hosted Amazon S3 bucket.
After you download this .pem file, you can import the file into AWS DMS as described following.
AWS Management Console
To import the public key (.pem) file
Open the AWS DMS console at https://console.aws.amazon.com/dms.
In the navigation pane, choose Certificates.
Choose Import certificate and do the following:
For Certificate identifier, enter a unique name for the certificate, for example docdb-cert.
For Import file, navigate to the location where you saved the .pem file.
When the settings are as you want them, choose Add new CA certificate.