This ‘conversion’ error happens only on IT3b or RAF documents. These are PDF documents issued by a bank or a fund. These documents are encrypted and password protected, but it’s not visible to you as the user.
That means if you open the document, it doesn’t prompt you for a password. However it’s still built into the document.
You should always print these documents, scan them and upload the scanned document to eFiling (not the original document).