Fix 'download' audit log entry when main blob is a PDF

When the main blob of a document is a PDF, the previewer in the Web UI (pdf.js) uses the nxfile URL to download the blob (from data field in property file:content in the JSON representation of the document). The problem is it insert a download audit log entry in the document's history, which is normally used to log a regular download, not a preview.

A possible solution would be to use the @rendition/pdf web adapter instead.