Download an extension that supports running user scripts (I prefer ViolentMonkey on Chrome-based browsers), add the script below as a new script and that’s it.

Go to a video (URL looks like this: https://www.pornhubpremium.com/view_video.php?viewkey=xxxxxxxxxxxxx), and you should see a new tab labelled “Download (GM)” together with the other tabs below the video (About, Share, and so on.)

Copy the title of the video (so you can easily paste it when beginning the download in the next step.)

Click the tab and you should see a list of download links (one for each resolution that’s available), right click one of them and select “Save as…” or whatever it’s called in your browser. Paste the title you copied into the file name box, then click Save/OK or whatever.

The video will be downloaded just like any other file download.

// ==UserScript==
// @name        PH Downloader - pornhubpremium.com
// @namespace   Violentmonkey Scripts
// @match       https://www.pornhubpremium.com/view_video.php
// @match       https://www.pornhub.com/view_video.php*
// @match       *://www.pornhub.com/view_video.php*
// @match       *://*.pornhub.com/view_video.php*
// @match       *://pornhub.com/view_video.php*
// @match       https://www.pornhubpremium.com/view_video.php*
// @match       *://www.pornhubpremium.com/view_video.php*
// @match       *://*.pornhubpremium.com/view_video.php*
// @match       *://pornhubpremium.com/view_video.php*
// @grant       none
// @version     1.0
// @author      -
// @description 3/12/2020, 10:56:35 PM
// ==/UserScript==

// First find the name of the flashvar variable in one of the scripts on the page:
var start = document.documentElement.innerHTML.indexOf('var flashvars_') + 'var '.length;
var tempDoc = document.documentElement.innerHTML.substr(start);
var end = tempDoc.indexOf(' =');
var flashVarsName = tempDoc.substr(0, end);
var flashVars = unsafeWindow[flashVarsName];


// Create a new tab element on the page:
var tabs = document.getElementsByClassName('video-actions-tabs')[0];
var tab = document.createElement('div');
tab.className = 'video-action-tab my-custom-tab';
tabs.appendChild(tab);

var title = document.createElement('div');
title.className = 'title';
title.innerText = 'Use right click and "Save as..." to download.';
tab.appendChild(title);

for(var i = 0; i < flashVars.mediaDefinitions.length; i++) {
  var definition = flashVars.mediaDefinitions[i];
  if (definition.format === 'hls') {
    continue;
  }
  
  console.log(definition);
  var container = document.createElement('div');
  var a = document.createElement('a');
  a.innerText = 'Download (' + definition.quality + 'p, .' + definition.format + ')';
  a.dataset.videoUrl = definition.videoUrl;
  a.href = definition.videoUrl;
  a.download = 'test-' + definition.quality + '.' + definition.format;
  a.target = '_blank';
  
  a.style.fontSize = '140%';
  container.style.marginBottom = '6px';
  
  container.appendChild(a);
  tab.appendChild(container);
}


// Create a new menu item, which opens the new tab:
var menu = document.getElementsByClassName('tab-menu-wrapper-row')[0];
var newItem = document.createElement('div');
var sub = document.createElement('div');
var icon = document.createElement('i');
var text = document.createElement('span');
text.innerText = 'Download (GM)';
icon.className = 'main-sprite-dark-2';
sub.className = 'tab-menu-item tooltipTrig';
sub.dataset.title = 'Download (GM)';
sub.dataset.tab = 'my-custom-tab';
sub.appendChild(icon);
sub.appendChild(text);
newItem.className = 'tab-menu-wrapper-cell';
newItem.appendChild(sub);
menu.appendChild(newItem);