WhatsApp has introduced the 'Message Yourself' feature for all users.
This feature allows users to save important files, documents, photos, videos, etc. without using a third-party app.
To message yourself on WhatsApp, ensure that your app is updated. Open the app, tap the New Chat icon, select 'Message Yourself,' and send the messages/files you want to save.
Alternatively, if the new feature is not available, you can create a new group with yourself as the only participant and send files as messages.