function processDocuments() {
  const ss = SpreadsheetApp.getActiveSpreadsheet();
  const customersSheet = ss.getSheetByName(CUSTOMERS_SHEET_NAME);
  const productsSheet = ss.getSheetByName(PRODUCTS_SHEET_NAME);
  const transactionsSheet = ss.getSheetByName(TRANSACTIONS_SHEET_NAME);
  const invoicesSheet = ss.getSheetByName(INVOICES_SHEET_NAME);
  const invoiceTemplateSheet = ss.getSheetByName(INVOICE_TEMPLATE_SHEET_NAME);

  // Gets data from the storage sheets as objects.
  const customers = dataRangeToObject(customersSheet);
  const products = dataRangeToObject(productsSheet);
  const transactions = dataRangeToObject(transactionsSheet);

  ss.toast('Creating Invoices', APP_TITLE, 1);
  const invoices = [];

  // Iterates for each customer calling createInvoiceForCustomer routine.
  customers.forEach(function (customer) {
    ss.toast(`Creating Invoice for ${customer.customer_name}`, APP_TITLE, 1);
    let invoice = createInvoiceForCustomer(
      customer, products, transactions, invoiceTemplateSheet, ss.getId());
    invoices.push(invoice);
  });
  // Writes invoices data to the sheet.
  invoicesSheet.getRange(2, 1, invoices.length, invoices[0].length).setValues(invoices);
}
Iterates through the worksheet data populating the template sheet with 
customer data, then saves each instance as a PDF document.

Called by user via custom menu item.