Outbox is an optional item that is hidden by default.
Here's the screenshot of Acrobat Reader for iOS. The Android version has a different user interface.
You will see the Outbox option only when you save a PDF document to Adobe Document Cloud or convert a document from/to PDF using a paid online subscription service (e.g. Adobe Export PDF, Adobe PDF Pack).
Your documents are stored in one of the following locations.
You can select Local to find the documents on your device and select Document Cloud/Create Cloud to find the documents in the cloud storage.
Hope this helps.