For users that are opted in telemetry reporting, it can go up to 40M PDF views per browser version [1]

  [1] http://telemetry.mozilla.org/dist.html#!cumulative=0&end_date=2015-06-05&max_channel_version=beta%252F38&measure=PDF_VIEWER_USED&min_channel_version=nightly%252F38&product=Firefox&sanitize=1&start_date=2015-03-04&trim=1&use_submission_date=0