Use the data creator app's high-level export/report functionality - one last time if the application will be decommissioned - to get all the data out aggregated. It's usually a reliable way, assuming it's a mature product.
Then we get
Aggregated and structured data
Likely we need to transform this further to be able present this to the user, but it's already aggregated, so it's easy.
Aggregated and unstructured data
The report tool could produce PDFs, which is human-readable, but searchability is terrible. We need at least some metadata on top of the documents.