Showing documents from Google Drive on webpage

Question 1

For HTML/JavaScript solution, look at the following links:

https://developers.google.com/drive/quickstart-js
https://www.youtube.com/watch?v=09geUJg11iA
https://developers.google.com/drive/web/auth/web-client

Here's the simplest way using JavaScript, most of the complexity is in your WebApp authorization. The example below reads files IDs, names and description in a folder you specify.
- go to: https://cloud.google.com/console/project and create a new project "xyz"
- Select "APIs & auth", disable the ones you don't need, enable "Drive API"
- Select "Credentials",
push "CREATE NEW CLIENT ID" button
x Web Application
Authorized Javascript origins: "https://googledrive.com/"
Authorized redirect URI: "https://googledrive.com/oauth2callback"

it will result in:
Client ID: xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx.apps.googleusercontent.com
Email address: xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx@developer.gserviceaccount.com
Client secret: xxxxxxxxxxxxxxxxxxxx
Redirect URIs: https://googledrive.com/oauth2callback
Javascript Origins: https://googledrive.com/

- in the code below, replace
CLIENT_ID with xxxxxxxxxxxxxxxxxxxxxxxxx.apps.googleusercontent.com
FOLDER_ID with the ID you see in the folder address line,
https://drive.google.com/?tab=mo&authuser=0#folders/xxxxxxxxxxxxxxxxxxx

- run it, authorize

I don't know if you read JS, the code can be followed from bottom up, I made is as simple as possible.

<script src="//ajax.googleapis.com/ajax/libs/jquery/1.7.2/jquery.min.js"></script>
<script>
  var FOLDER_ID = '.xxxxxxxxxxxxxxxxxx';    // the folder files reside in
  var CLIENT_ID = 'xxxxxxxxxxxxxxxxxxxxxxxxxxxx.apps.googleusercontent.com';
  var SCOPE =    //'https://www.googleapis.com/auth/drive'; 
  [
    'https://www.googleapis.com/auth/drive',
    'https://www.googleapis.com/auth/drive.file',     // for description, 
  ];

  function rsvpCB(resp) {
    var picAlbumLst = '<ul>\n';
    for (i=0; i<resp.items.length; i++) 
      picAlbumLst += (
      '  <li>'+resp.items[i].id+',&nbsp;'+resp.items[i].title+',&nbsp;'+resp.items[i].description+'</li>\n');
    picAlbumLst += "</ul>\n";
    $('#container').append(picAlbumLst);
  }
  function rqstCB() {   //test @ https://developers.google.com/drive/v2/reference/files/list
    var rv = gapi.client.drive.files.list({
      'q': '"'+FOLDER_ID+'" in parents and trashed = false',
      'fields' : 'items(id,title,description)'   //'items(id,title,description,indexableText)'   
    }).execute(rsvpCB);
  }
  // authorization server reply
  function onAuthResult(authResult) {
    var authButton = document.getElementById('authorizeButton');
    authButton.style.display = 'none';
    if (authResult && !authResult.error) {  // access token successfully retrieved
      gapi.client.load('drive', 'v2', rqstCB);   
    } else {  // no access token retrieved, force the authorization flow.
      authButton.style.display = 'block';
      authButton.onclick = function() {
        checkAuth(false);
      }
    }
  }
  // check if the current user has authorized the application.
  function checkAuth(bNow) {
    gapi.auth.authorize({'client_id':CLIENT_ID, 'scope':SCOPE, 'immediate':bNow}, onAuthResult);
  }
  // called when the client library is loaded, look below
  function onLoadCB() { 
    checkAuth(true); 
  }
</script>
<script src="https://apis.google.com/js/client.js?onload=onLoadCB"></script>
<body style="background-color: transparent;">
  <input type="button" id="authorizeButton" style="display: none" value="Authorize" />
  <div id="container">
  </div>
</body>

Question 2

The fastest and easiest solution is to embed the folder using an iframe (no javascript needed). Obviously this is also the least flexible solution, although you can use CSS to change the layout of the iframe contents (see below).

Google Drive won't allow embedding of the url you would normally use. It has its X-Frame-Options header set to "SAMEORIGIN", preventing use in an iframe. So you have to use the following link, which will allow embedding:
https://drive.google.com/embeddedfolderview?id=DOCUMENT_ID#VIEW_TYPE

DOCUMENT_ID is the id that is mentioned in the normal share link (which looks like https://drive.google.com/folderview?id=DOCUMENT_ID), so you can just copy that from there.

VIEW_TYPE should be either 'grid' or 'list', depending on your preference.

And if you need to change the style of the iframe content, take a look at this solution.

Question 3

This should be done with Google API. You can search google drive php api list files on google. And also I found this and this on SO.

Question 4

Here are some main points:

Do you want anyone with the URL to be able to see your document? You can share a document as public to anyone on the internet. Plus you can set read access to specific folders. Just right click a Google Doc file, and choose 'Share' from the short cut menu.
I'm assuming you want people to download your docs, even when you are not signed in. This is called 'Offline Access', and is one of many terms you'll need to figure out in order to do all of this with a program.
If you only want to give read access to the user, using JavaScript, jQuery, etc on the front end is a viable option. You can also do this in PHP, it's just a matter of personal preference.
To do all of this in code, you need to grant authorization to read your files. The oAuth2 process has multiple steps, and it's good to understand the basic flow. Setting up the code and the webpages to initially grant authorization, then retrieve and store the tokens can get confusing.
Your Google Project has a setting for where the origin of the authorization request is coming from. That is your website. But if you want to develop and test locally, you can set the Javascript Origins to http://localhost
How much time do you have, and how much programming experience? Would it be easier to give the user a few lines of instruction to "Manually" download your file, rather than program the authorization check?
Putting the document into your webpage is the easy part.
In order to embed a Google doc in your website, go to your Google Drive, open a document and choose File then Publish to Web, and you will be given an HTML iFrame Tag that can be embedded into you web page. You can change the height and width of the iFrame to match the document size. iFrame Instructions W3Schools
Downloading your document can be done very easily from the online version of a shared document just by choosing FILE and then DOWNLOAD AS from the menu.
To get up and running fast, just give the user a couple lines of instructions on how to download "Manually", then see if you can program the code.
Provide a link to your shared document instead of programming the button, and then work on the code.
Search Git Hub for Google Drive, you might find something there.
Some of the official Google code examples are way more complicated than you need, and will take a long time to figure out. The code examples in the documentation pages are simpler, but are almost never complete functioning code examples. You'll need to put lots of pieces of the puzzle together to make it work.