It appears that the pdfjs.database you found is actually a list of previously opened pdfs and their saved pdf view settings including last read page number and zoom level as well as a couple of other view settings.
On my machine this is a patial display of some of the contents of that key.
{"files": [{"fingerprint":"7a798210f6814668a779da60662f2c6d","exists":true,"page":4,"zoom":"auto","scrollLeft":0,"scrollTop":617},
{"fingerprint":"604c4e776174cb1f9b3f835f566827","exists":true,"page":2,"zoom":"150","scrollLeft":612,"scrollTop":792},
{"fingerprint":"fb6578d44ae9f8f346bd67d57a2d8","exists":true,"page":1,"zoom":"auto","scrollLeft":0,"scrollTop":792}]}
Opening a new pdf and changing the view setting simply added a new item to the end of the list.
While changing the Zoom to 150 on all those items will affect documents you have already opened I do not believe you can set this up for new pdfs.
It appears this feature is known but not being actively worked on. Also see this drupal page.
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