The `el` property of PDF pages was removed from PDF.js in February 2015
in eed67ea8bbba61a05877a99dc2f03b5adad5a288 so we can remove this check
while still supporting older versions of PDF.js that we care about.

Also rewrite an unnecessarily complex loop header. The `pagesCount`
property cannot be negative, so there is no need to check for this.