The documents you uploaded are most likely stored in Salesforce Classic Documents, which is an older feature that Salesforce no longer fully supports in Lightning Experience. While Apsona can still access and use these documents for mail merges, Lightning UI does not display Classic Documents, which is why they seem to "disappear" after upload.
To view or manage these documents, you’ll need to switch to Salesforce Classic. Once there, navigate to Documents via the "All Tabs" (+) menu, and search by the document name or folder.
If needed, you can re-upload the documents and organize them within a specific folder. For future uploads, consider using Salesforce Files or a Lightning-compatible storage method if you'd like consistent visibility in both Lightning and Classic.