I discovered that the “Quickbooks PDF Converter 2.0” was assigned a port of LPT1 while Quicken or Quickbooks was assigned a port of “nul:”. Once I reassigned the port for the “Quickbooks PDF Converter 2.0” to “nul:”, Quickbooks no longer hung attempting to send the invoices.
To re-assign the port for the “Quickbooks PDF Converter 2.0”, locate you the Quickbooks PDF Converter in your list of printers, and select the properties. On the printer properties pop-up, there is a tab called ports. If it is assigned to a port other than “nul:”, re-assign it to “nul:” and click the “Apply” button. This configuration change fixed the problem for me.