Versions are a way to keep track of multiple variations of the document, when you edit a sent document and/or upload a new document (in the case when you create documents via uploading a file like .pdf).
You can download a version for your records.
Versions for uploaded document
Open an existing document, click "Edit" on the right. If the document is in the Sent status, you will see options for “Rearrange Fields” or “Upload a new Source file”. If it's still a Draft, you will see the option to upload a new source file only.
Choose one and make necessary changes.
Once you either changed fields layout or uploaded a new file, click More… > Versions.
You have the option to download a PDF of the new version or the previous one.
Versions for Editable document
Note:versions of documents created via the Content Builder are available if you edited a document after it was sent.
To access versions open the document and click More > Versions. You have the option to download a PDF of the new version or the previous one.