title: How to allow downloading file from web app?
body: <p>I will simplify my problem.</p>
  
  <p>Customers of my website can order a documentation (a PDF file). I have a full process that starts by parsing a mail containings the order.
  Once I was paid, a script sends an email to the customer with a link to the file to download. 
  The file is a copy of an original file stored on my Google Drive. Each customer has his own copy. The copy is deleted after a while (7 days).
  It works fine.
  But... I need to know if the customer have downloaded the file. 
  Of course, Google drive doesn't allow to track downloading. But I had an idea !</p>
  
  <p>Instead of including in the mail the link to the file to download, I want to include a link to a Google WebApp. The link will have just the id file to download.
  When the customer will click on the link, the webapp will read the file id in the url parameters. Then, its' quite easy to build the final URL to download the file :</p>
  
  <p><a href="https://drive.google.com/uc?export=download&amp;id=#####" rel="nofollow noreferrer">https://drive.google.com/uc?export=download&amp;id=#####</a></p>
  
  <p>My script will esasily track the download in a spread sheet file I already use. That's my solution.</p>
  
  <p>But, the idea is not all :(. I found a few scripts to download files from a webapp. But I think there are to evoluated (or far away from my need).</p>
  
  <p>I think I must have a glance to the doGet() function. I can get parameters (so the file id) and, as explained above, build the final downloadable file URL. But, how can I send this new URL to the customer bowser to urge it to download the file ? Note that the PDF file is rather big (over 50 Mo) : it may be important for the solution.</p>
  
  <p>Thanks for your help !</p>