A snippet to enable the PDF file preview by using 'filePreview.js'.

sp-pdfpreview.js

/**
 * sp-pdfpreview.js
 *
 * (c) 2017 koohq. Licensed under CC0.
 * https://creativecommons.org/publicdomain/zero/1.0/legalcode
 */
(function enablePDFPreview() {
  ExecuteOrDelayUntilScriptLoaded(function() {
    filePreviewManager.previewers.extensionToPreviewerMap.pdf = [embeddedWACPreview, WACImagePreview];
    embeddedWACPreview.dimensions.pdf = { width: 379, height: 252 };
  }, 'filePreview.js');
})();