Stripe, users can not download their invoice

I am working with Stripe Payment Gateway and everything is working so far, except the option, that my users can download their invoice on my website. It is possible to download an invoice inside the stripe dashboard but that is not what I need. After my users made a payment, they should be able to download their invoices to my system.

Currently I am working with stripe webhooks. A user makes a payment and the data is saved inside my database. This is how my code looks like:

StripeStripe::setApiKey("$stripe_secret_key_test");
// Retrieve the request's body and parse it as JSON:
$input = @file_get_contents('php://input');
$event_json = json_decode($input);

// Update database with new billing entry
if ($event_json->type == 'invoice.payment_succeeded') {
    $invoice_id = $event_json->data->object->id;
    $customer = $event_json->data->object->customer;
    $date = $event_json->data->object->date;
    $price = $event_json->data->object->amount_paid;
    $user = $event_json->data->object->lines->data[0]->metadata->userid;
    $stripe_sub_id = $event_json->data->object->lines->data[0]->id;
    $plan_id = $event_json->data->object->lines->data[0]->plan->id;
    $stamp_created = $event_json->data->object->lines->data[0]->plan->created;
    $comments = $event_json->data->object->lines->data[0]->plan->nickname;
    $period_start = $event_json->data->object->lines->data[0]->period->start;
    $period_end = $event_json->data->object->lines->data[0]->period->end;

    $content=array("invoice"=>$invoice_id,
        "user"=>$user,
        "stripe_cust_id"=>$customer,
        "stripe_sub_id"=>$stripe_sub_id,
        "idea_id"=>0,
        "billing_date"=>$date,
        "plan"=>$plan_id,
        "price"=>$price,
        "stamp_created"=>$stamp_created,
        "period_start"=>$period_start,
        "period_end"=>$period_end,
        "status"=>'paid',
        "printed"=>0);
    insertGWInfo('billing',$content);

    $content=array("user"=>$user,
        "trans_time"=>date("Y-m-d H:i:s"),
        "ip"=>$_SERVER['REMOTE_ADDR'],
        "trans_type"=>1,
        "cash_amount"=>$price,
        "comments"=>"Subscription for ".$comments."",
        "idea_id"=>0);
    insertGWInfo('user_accounting',$content);
}

Now I´ve created a table where I fetch those entries from my database and display them to my customer. I would like to have a button now with “Download Invoice” where my customers can download the subscription invoice which appears monthly.

Any idea how I can do this? I have the invoice_id but where do I get the URL to download an invoice? I wasn´t able to find anything inside the stripe docs. Any help would be really appreciated.

EDIT: Here is the JSON outcome for invoice.payment_suceeded:

"{
  "created": 1326853478,
  "livemode": false,
  "id": "evt_00000000000000",
  "type": "invoice.payment_succeeded",
  "object": "event",
  "request": null,
  "pending_webhooks": 1,
  "api_version": "2018-02-28",
  "data": {
    "object": {
      "id": "in_00000000000000",
      "object": "invoice",
      "amount_due": 1000,
      "amount_paid": 1000,
      "amount_remaining": 0,
      "application_fee": null,
      "attempt_count": 1,
      "attempted": true,
      "billing": "charge_automatically",
      "charge": "_00000000000000",
      "closed": true,
      "currency": "eur",
      "customer": "cus_00000000000000",
      "date": 1526455801,
      "description": null,
      "discount": null,
      "due_date": null,
      "ending_balance": 0,
      "forgiven": false,
      "lines": {
        "data": [
          {
            "id": "sub_Cs4DUPJKcAJ7cg",
            "object": "line_item",
            "amount": 1000,
            "currency": "eur",
            "description": "1 standard-inv × Standard Investor (at €10.00 / month)",
            "discountable": true,
            "livemode": false,
            "metadata": {
            },
            "period": {
              "end": 1531726201,
              "start": 1529134201
            },
            "plan": {
              "id": "3",
              "object": "plan",
              "aggregate_usage": null,
              "amount": 1000,
              "billing_scheme": "per_unit",
              "created": 1526375755,
              "currency": "eur",
              "interval": "month",
              "interval_count": 1,
              "livemode": false,
              "metadata": {
              },
              "nickname": "Standard Plan for Investors",
              "product": "prod_CrihTVfgRHgImD",
              "tiers": null,
              "tiers_mode": null,
              "transform_usage": null,
              "trial_period_days": null,
              "usage_type": "licensed"
            },
            "proration": false,
            "quantity": 1,
            "subscription": null,
            "subscription_item": "si_Cs4DrvYN2FlCbS",
            "type": "subscription"
          }
        ],
        "has_more": false,
        "object": "list",
        "url": "/v1/invoices/in_1CSK5FDsOByhb3e8m8Z1BooY/lines"
      },
      "livemode": false,
      "metadata": {
      },
      "next_payment_attempt": null,
      "number": "774C629-0001",
      "paid": true,
      "period_end": 1526455801,
      "period_start": 1526455801,
      "receipt_number": null,
      "starting_balance": 0,
      "statement_descriptor": null,
      "subscription": "sub_00000000000000",
      "subtotal": 1000,
      "tax": null,
      "tax_percent": null,
      "total": 1000,
      "webhooks_delivered_at": 1526455810
    }
  }
}"

Answer

Stripe does not support downloading the invoice as a PDF at the moment via the API and it is only available in the dashboard instead.

For now, the best solution is to build your own invoice receipt/pdf instead. You can retrieve the Invoice object via the API which tells you what the invoice is for: period, amount, when it was paid, what’s in that invoice, etc. And you can then generate your own receipt for your customers on your end.