You pass the same parameters (via DocumentOptions) to the DocumentViewer as you passed to DocumentCache then viewer "knows" it's the same document and loads it from the cache. So if you used DocumentOptions.Document property then you set the same value for DocumentViewer.
For example for files on disk, the cache unique ID is calculated as a string combination of file extension, file size and file date . This way cache collisions do not occur and we can reuse the cached file even if the file name before extension is changed (because it's still the same document).
If you provided a custom unique ID, then provide the same to DocumentViewer.